<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2022 Workiva-->
<!--r:eaab86ae-acc3-45b1-8aea-0f8ccb2e7e72,g:f545e7b2-6644-4a6e-834d-389954432ed8-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.ducommun.com/role/CondensedConsolidatedStatementsofChangesinShareholdersEquity" xlink:type="simple" xlink:href="dco-20220402.xsd#CondensedConsolidatedStatementsofChangesinShareholdersEquity"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/CondensedConsolidatedStatementsofChangesinShareholdersEquity" xlink:type="extended" id="i720af436678f4d978f5add926729c50f_CondensedConsolidatedStatementsofChangesinShareholdersEquity">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_390a1e2e-41e7-41ce-a2a2-ac58a29c9017" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_390a1e2e-41e7-41ce-a2a2-ac58a29c9017" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_57115f04-6067-4d94-96b9-615b86958e15" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:to="loc_us-gaap_SharesOutstanding_57115f04-6067-4d94-96b9-615b86958e15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_6101760d-7dfe-4668-be6d-02e79152854d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:to="loc_us-gaap_StockholdersEquity_6101760d-7dfe-4668-be6d-02e79152854d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_ffab0ce8-4e27-4899-9dae-6f74666fc65b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:to="loc_us-gaap_NetIncomeLoss_ffab0ce8-4e27-4899-9dae-6f74666fc65b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_e95d862b-35d5-43ac-84d6-87b46eba4a84" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_e95d862b-35d5-43ac-84d6-87b46eba4a84" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_63b5c88a-de4a-4544-b1ab-c5a6b8ba14dd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_63b5c88a-de4a-4544-b1ab-c5a6b8ba14dd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_9a76b07a-5d91-4401-b4ed-6100c678e870" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_9a76b07a-5d91-4401-b4ed-6100c678e870" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_e9fd0723-1373-45d3-9e50-13f1372a98c7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_e9fd0723-1373-45d3-9e50-13f1372a98c7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_f10a1bac-9808-4eb5-9769-e7c0e0303634" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_f10a1bac-9808-4eb5-9769-e7c0e0303634" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_f6ff311e-a226-4dbc-8f6a-db1b57c0d72e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_f6ff311e-a226-4dbc-8f6a-db1b57c0d72e" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_8e4d76ef-7871-4499-8f43-d7e05c5c763a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_8e4d76ef-7871-4499-8f43-d7e05c5c763a" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodShares_2b029a43-1da9-4473-ae35-eb239ee84eb2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodShares_2b029a43-1da9-4473-ae35-eb239ee84eb2" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodValue_7f4aee9a-48fe-41ec-a877-02fe92e76930" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodValue_7f4aee9a-48fe-41ec-a877-02fe92e76930" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_40e731bb-e255-403e-abb4-e2e41f1fe3cf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_e2fd628f-03e2-4e84-ad88-fede9bae34be" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_40e731bb-e255-403e-abb4-e2e41f1fe3cf" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_19596590-a46f-4659-a62a-2a6d0d74356e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_6ddb741b-8ebe-4fd7-ba37-a74fc87a0510" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_6ed31043-77f0-4352-8f52-fcee75ba941d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_390a1e2e-41e7-41ce-a2a2-ac58a29c9017" xlink:to="loc_us-gaap_StatementTable_6ed31043-77f0-4352-8f52-fcee75ba941d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_3c351199-1b1f-4e4e-a490-1ff3f82e70ae" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_6ed31043-77f0-4352-8f52-fcee75ba941d" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_3c351199-1b1f-4e4e-a490-1ff3f82e70ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_3c351199-1b1f-4e4e-a490-1ff3f82e70ae_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_3c351199-1b1f-4e4e-a490-1ff3f82e70ae" xlink:to="loc_us-gaap_EquityComponentDomain_3c351199-1b1f-4e4e-a490-1ff3f82e70ae_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_d6ac2761-7c7a-4a1d-91c8-bc675f24497a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_3c351199-1b1f-4e4e-a490-1ff3f82e70ae" xlink:to="loc_us-gaap_EquityComponentDomain_d6ac2761-7c7a-4a1d-91c8-bc675f24497a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_7221aeb8-7ec1-4af2-bb3a-bc76586a0b61" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_d6ac2761-7c7a-4a1d-91c8-bc675f24497a" xlink:to="loc_us-gaap_CommonStockMember_7221aeb8-7ec1-4af2-bb3a-bc76586a0b61" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_05a9efd1-17b4-4de9-baac-c80c79c4047b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_d6ac2761-7c7a-4a1d-91c8-bc675f24497a" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_05a9efd1-17b4-4de9-baac-c80c79c4047b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_a8c6609f-a4e9-4559-a615-a45aee89fda9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_d6ac2761-7c7a-4a1d-91c8-bc675f24497a" xlink:to="loc_us-gaap_RetainedEarningsMember_a8c6609f-a4e9-4559-a615-a45aee89fda9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_c9f0dcd8-2395-4b93-8279-8b0324b5b1c7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_d6ac2761-7c7a-4a1d-91c8-bc675f24497a" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_c9f0dcd8-2395-4b93-8279-8b0324b5b1c7" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/SummaryofSignificantAccountingPoliciesNarrativeDetails" xlink:type="simple" xlink:href="dco-20220402.xsd#SummaryofSignificantAccountingPoliciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/SummaryofSignificantAccountingPoliciesNarrativeDetails" xlink:type="extended" id="i67000aea19e44937ae541e1d0844a7bc_SummaryofSignificantAccountingPoliciesNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:href="dco-20220402.xsd#dco_SignificantAccountingPoliciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_6ab071d9-0c99-4412-88fa-ca331b2ae5e0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_NumberOfReportableSegments_6ab071d9-0c99-4412-88fa-ca331b2ae5e0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringAndRelatedCostExpectedCost1_f89eac46-bdfe-4726-aa5e-7131149736da" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringAndRelatedCostExpectedCost1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_RestructuringAndRelatedCostExpectedCost1_f89eac46-bdfe-4726-aa5e-7131149736da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax_1ef6e367-3bff-45f3-bfa4-00432e4c99ba" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax_1ef6e367-3bff-45f3-bfa4-00432e4c99ba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetAccumulatedAllowanceForCreditLoss_0d2bb818-b737-4098-971c-586bdc019015" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerAssetAccumulatedAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_ContractWithCustomerAssetAccumulatedAllowanceForCreditLoss_0d2bb818-b737-4098-971c-586bdc019015" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_ProductionCostOfContractsCurrent_e503e51a-a29d-48d5-bd98-5ffbbbc8128a" xlink:href="dco-20220402.xsd#dco_ProductionCostOfContractsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_dco_ProductionCostOfContractsCurrent_e503e51a-a29d-48d5-bd98-5ffbbbc8128a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_16e3bf16-1d60-4656-91a0-53bae87c46f9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_16e3bf16-1d60-4656-91a0-53bae87c46f9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_02c9c787-81d3-4b02-a51d-6349f5ba5701" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_02c9c787-81d3-4b02-a51d-6349f5ba5701" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_c4ba60cd-380f-4fe0-9f8d-9f6301075a81" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_c4ba60cd-380f-4fe0-9f8d-9f6301075a81" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_67bb79b2-11d0-4b9c-839f-399a5a0f3531" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_67bb79b2-11d0-4b9c-839f-399a5a0f3531" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrantsReceivable_d8e34438-1031-4dff-991b-03d2579da7bc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GrantsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_GrantsReceivable_d8e34438-1031-4dff-991b-03d2579da7bc" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_ProceedsReceivedFromGrants_ac54bf6b-aeb3-4b86-9371-5d8d531b5eee" xlink:href="dco-20220402.xsd#dco_ProceedsReceivedFromGrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_dco_ProceedsReceivedFromGrants_ac54bf6b-aeb3-4b86-9371-5d8d531b5eee" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrantsReceivableCurrent_bec4d4b4-5c78-4518-9b12-6faf02964319" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GrantsReceivableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_GrantsReceivableCurrent_bec4d4b4-5c78-4518-9b12-6faf02964319" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_GovernmentGrantExpense_6fe69e1c-11ab-40d9-8f08-ede04e645045" xlink:href="dco-20220402.xsd#dco_GovernmentGrantExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_dco_GovernmentGrantExpense_6fe69e1c-11ab-40d9-8f08-ede04e645045" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesAndOtherLiabilities_0d4feb54-6ea4-4f1d-bc0b-ec767b42058d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccruedLiabilitiesAndOtherLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_AccruedLiabilitiesAndOtherLiabilities_0d4feb54-6ea4-4f1d-bc0b-ec767b42058d" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:href="dco-20220402.xsd#dco_SignificantAccountingPoliciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_06071af5-1eb3-49b3-84d4-b75495f8cce3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_06071af5-1eb3-49b3-84d4-b75495f8cce3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_06071af5-1eb3-49b3-84d4-b75495f8cce3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_06071af5-1eb3-49b3-84d4-b75495f8cce3" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_06071af5-1eb3-49b3-84d4-b75495f8cce3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_1fecbca4-1cbc-400b-8669-3f89e516f7fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_06071af5-1eb3-49b3-84d4-b75495f8cce3" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_1fecbca4-1cbc-400b-8669-3f89e516f7fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_e7d98393-9325-48ea-99a4-76ca6658a026" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_1fecbca4-1cbc-400b-8669-3f89e516f7fd" xlink:to="loc_us-gaap_SubsequentEventMember_e7d98393-9325-48ea-99a4-76ca6658a026" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_5cfe65d9-cfb3-4dcc-9cc7-59ebfdd0ac6e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:to="loc_us-gaap_HedgingDesignationAxis_5cfe65d9-cfb3-4dcc-9cc7-59ebfdd0ac6e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_5cfe65d9-cfb3-4dcc-9cc7-59ebfdd0ac6e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_5cfe65d9-cfb3-4dcc-9cc7-59ebfdd0ac6e" xlink:to="loc_us-gaap_HedgingDesignationDomain_5cfe65d9-cfb3-4dcc-9cc7-59ebfdd0ac6e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_866de49b-897b-4c44-988a-26e13e503e7c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_5cfe65d9-cfb3-4dcc-9cc7-59ebfdd0ac6e" xlink:to="loc_us-gaap_HedgingDesignationDomain_866de49b-897b-4c44-988a-26e13e503e7c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_61cd500f-6c3f-4424-b534-229f94a32289" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_866de49b-897b-4c44-988a-26e13e503e7c" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_61cd500f-6c3f-4424-b534-229f94a32289" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_aa08437a-c091-4e85-a652-cebe900b6288" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_aa08437a-c091-4e85-a652-cebe900b6288" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_0ca76bcb-d649-402f-98c2-1c17047f8fab" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_0ca76bcb-d649-402f-98c2-1c17047f8fab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_0ca76bcb-d649-402f-98c2-1c17047f8fab_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_0ca76bcb-d649-402f-98c2-1c17047f8fab" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_0ca76bcb-d649-402f-98c2-1c17047f8fab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_940116e0-051a-496f-9df1-b9de4a733185" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_0ca76bcb-d649-402f-98c2-1c17047f8fab" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_940116e0-051a-496f-9df1-b9de4a733185" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_c29226ca-d4eb-46da-826f-0a973770b2a9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_940116e0-051a-496f-9df1-b9de4a733185" xlink:to="loc_us-gaap_CostOfSalesMember_c29226ca-d4eb-46da-826f-0a973770b2a9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_d28ac15b-66bd-4f2c-a2de-10609b032a8c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_940116e0-051a-496f-9df1-b9de4a733185" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_d28ac15b-66bd-4f2c-a2de-10609b032a8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_c3d177b7-165b-4976-b52e-e65b02364e91" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:to="loc_srt_RangeAxis_c3d177b7-165b-4976-b52e-e65b02364e91" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_c3d177b7-165b-4976-b52e-e65b02364e91_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_c3d177b7-165b-4976-b52e-e65b02364e91" xlink:to="loc_srt_RangeMember_c3d177b7-165b-4976-b52e-e65b02364e91_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_b944289c-6e71-4dc1-8b19-12fb6f0adff2" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_c3d177b7-165b-4976-b52e-e65b02364e91" xlink:to="loc_srt_RangeMember_b944289c-6e71-4dc1-8b19-12fb6f0adff2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_dd1c9155-37db-4f53-a2f1-1b9000c21fdc" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b944289c-6e71-4dc1-8b19-12fb6f0adff2" xlink:to="loc_srt_MinimumMember_dd1c9155-37db-4f53-a2f1-1b9000c21fdc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_97d3af74-a3e3-484c-9c6c-6b38c7f7a42e" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b944289c-6e71-4dc1-8b19-12fb6f0adff2" xlink:to="loc_srt_MaximumMember_97d3af74-a3e3-484c-9c6c-6b38c7f7a42e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_f7e31c2b-b093-43e8-81b3-6eb37ab71d2e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_f7e31c2b-b093-43e8-81b3-6eb37ab71d2e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_f7e31c2b-b093-43e8-81b3-6eb37ab71d2e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_f7e31c2b-b093-43e8-81b3-6eb37ab71d2e" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_f7e31c2b-b093-43e8-81b3-6eb37ab71d2e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_43caac34-dc92-4be4-b561-32404bff9312" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_f7e31c2b-b093-43e8-81b3-6eb37ab71d2e" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_43caac34-dc92-4be4-b561-32404bff9312" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeSeveranceMember_d461a43d-d7eb-4c6e-a632-40af1f1211f7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeSeveranceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_43caac34-dc92-4be4-b561-32404bff9312" xlink:to="loc_us-gaap_EmployeeSeveranceMember_d461a43d-d7eb-4c6e-a632-40af1f1211f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherRestructuringMember_5cf5058c-bd4c-4844-9f2b-4ced477613ba" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherRestructuringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_43caac34-dc92-4be4-b561-32404bff9312" xlink:to="loc_us-gaap_OtherRestructuringMember_5cf5058c-bd4c-4844-9f2b-4ced477613ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanAxis_feee46eb-393d-4367-a850-4b490747d303" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringPlanAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:to="loc_us-gaap_RestructuringPlanAxis_feee46eb-393d-4367-a850-4b490747d303" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_feee46eb-393d-4367-a850-4b490747d303_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_feee46eb-393d-4367-a850-4b490747d303" xlink:to="loc_us-gaap_RestructuringPlanDomain_feee46eb-393d-4367-a850-4b490747d303_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_0fe62f77-0a04-429b-8c12-0c1b170a4029" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_feee46eb-393d-4367-a850-4b490747d303" xlink:to="loc_us-gaap_RestructuringPlanDomain_0fe62f77-0a04-429b-8c12-0c1b170a4029" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_April2022RestructuringPlanMember_5db20d37-02cf-4aed-a8ac-c30c12a205e6" xlink:href="dco-20220402.xsd#dco_April2022RestructuringPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_0fe62f77-0a04-429b-8c12-0c1b170a4029" xlink:to="loc_dco_April2022RestructuringPlanMember_5db20d37-02cf-4aed-a8ac-c30c12a205e6" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/SummaryofSignificantAccountingPoliciesNarrativeDetails_1" xlink:type="simple" xlink:href="dco-20220402.xsd#SummaryofSignificantAccountingPoliciesNarrativeDetails_1"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/SummaryofSignificantAccountingPoliciesNarrativeDetails_1" xlink:type="extended" id="i970ba2dffbef46148353019bd2be0eff_SummaryofSignificantAccountingPoliciesNarrativeDetails_1">
    <link:loc xlink:type="locator" xlink:label="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:href="dco-20220402.xsd#dco_SignificantAccountingPoliciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_6ab071d9-0c99-4412-88fa-ca331b2ae5e0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_NumberOfReportableSegments_6ab071d9-0c99-4412-88fa-ca331b2ae5e0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringAndRelatedCostExpectedCost1_f89eac46-bdfe-4726-aa5e-7131149736da" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringAndRelatedCostExpectedCost1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_RestructuringAndRelatedCostExpectedCost1_f89eac46-bdfe-4726-aa5e-7131149736da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax_1ef6e367-3bff-45f3-bfa4-00432e4c99ba" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax_1ef6e367-3bff-45f3-bfa4-00432e4c99ba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetAccumulatedAllowanceForCreditLoss_0d2bb818-b737-4098-971c-586bdc019015" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerAssetAccumulatedAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_ContractWithCustomerAssetAccumulatedAllowanceForCreditLoss_0d2bb818-b737-4098-971c-586bdc019015" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_ProductionCostOfContractsCurrent_e503e51a-a29d-48d5-bd98-5ffbbbc8128a" xlink:href="dco-20220402.xsd#dco_ProductionCostOfContractsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_dco_ProductionCostOfContractsCurrent_e503e51a-a29d-48d5-bd98-5ffbbbc8128a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_16e3bf16-1d60-4656-91a0-53bae87c46f9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_16e3bf16-1d60-4656-91a0-53bae87c46f9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_02c9c787-81d3-4b02-a51d-6349f5ba5701" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_02c9c787-81d3-4b02-a51d-6349f5ba5701" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_c4ba60cd-380f-4fe0-9f8d-9f6301075a81" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationPercentage_c4ba60cd-380f-4fe0-9f8d-9f6301075a81" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_67bb79b2-11d0-4b9c-839f-399a5a0f3531" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_67bb79b2-11d0-4b9c-839f-399a5a0f3531" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrantsReceivable_d8e34438-1031-4dff-991b-03d2579da7bc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GrantsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_GrantsReceivable_d8e34438-1031-4dff-991b-03d2579da7bc" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_ProceedsReceivedFromGrants_ac54bf6b-aeb3-4b86-9371-5d8d531b5eee" xlink:href="dco-20220402.xsd#dco_ProceedsReceivedFromGrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_dco_ProceedsReceivedFromGrants_ac54bf6b-aeb3-4b86-9371-5d8d531b5eee" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrantsReceivableCurrent_bec4d4b4-5c78-4518-9b12-6faf02964319" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GrantsReceivableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_GrantsReceivableCurrent_bec4d4b4-5c78-4518-9b12-6faf02964319" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_GovernmentGrantExpense_6fe69e1c-11ab-40d9-8f08-ede04e645045" xlink:href="dco-20220402.xsd#dco_GovernmentGrantExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_dco_GovernmentGrantExpense_6fe69e1c-11ab-40d9-8f08-ede04e645045" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesAndOtherLiabilities_0d4feb54-6ea4-4f1d-bc0b-ec767b42058d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccruedLiabilitiesAndOtherLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_us-gaap_AccruedLiabilitiesAndOtherLiabilities_0d4feb54-6ea4-4f1d-bc0b-ec767b42058d" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:href="dco-20220402.xsd#dco_SignificantAccountingPoliciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dco_SignificantAccountingPoliciesLineItems_41edc49b-3e0e-42db-b223-ccc611dfe9ad" xlink:to="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_06071af5-1eb3-49b3-84d4-b75495f8cce3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_06071af5-1eb3-49b3-84d4-b75495f8cce3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_06071af5-1eb3-49b3-84d4-b75495f8cce3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_06071af5-1eb3-49b3-84d4-b75495f8cce3" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_06071af5-1eb3-49b3-84d4-b75495f8cce3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_1fecbca4-1cbc-400b-8669-3f89e516f7fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_06071af5-1eb3-49b3-84d4-b75495f8cce3" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_1fecbca4-1cbc-400b-8669-3f89e516f7fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_e7d98393-9325-48ea-99a4-76ca6658a026" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_1fecbca4-1cbc-400b-8669-3f89e516f7fd" xlink:to="loc_us-gaap_SubsequentEventMember_e7d98393-9325-48ea-99a4-76ca6658a026" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_5cfe65d9-cfb3-4dcc-9cc7-59ebfdd0ac6e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:to="loc_us-gaap_HedgingDesignationAxis_5cfe65d9-cfb3-4dcc-9cc7-59ebfdd0ac6e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_5cfe65d9-cfb3-4dcc-9cc7-59ebfdd0ac6e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_5cfe65d9-cfb3-4dcc-9cc7-59ebfdd0ac6e" xlink:to="loc_us-gaap_HedgingDesignationDomain_5cfe65d9-cfb3-4dcc-9cc7-59ebfdd0ac6e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_866de49b-897b-4c44-988a-26e13e503e7c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_5cfe65d9-cfb3-4dcc-9cc7-59ebfdd0ac6e" xlink:to="loc_us-gaap_HedgingDesignationDomain_866de49b-897b-4c44-988a-26e13e503e7c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_61cd500f-6c3f-4424-b534-229f94a32289" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_866de49b-897b-4c44-988a-26e13e503e7c" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_61cd500f-6c3f-4424-b534-229f94a32289" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_0ca76bcb-d649-402f-98c2-1c17047f8fab" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_0ca76bcb-d649-402f-98c2-1c17047f8fab" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_0ca76bcb-d649-402f-98c2-1c17047f8fab_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_0ca76bcb-d649-402f-98c2-1c17047f8fab" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_0ca76bcb-d649-402f-98c2-1c17047f8fab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_940116e0-051a-496f-9df1-b9de4a733185" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_0ca76bcb-d649-402f-98c2-1c17047f8fab" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_940116e0-051a-496f-9df1-b9de4a733185" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_c29226ca-d4eb-46da-826f-0a973770b2a9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_940116e0-051a-496f-9df1-b9de4a733185" xlink:to="loc_us-gaap_CostOfSalesMember_c29226ca-d4eb-46da-826f-0a973770b2a9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_d28ac15b-66bd-4f2c-a2de-10609b032a8c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_940116e0-051a-496f-9df1-b9de4a733185" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_d28ac15b-66bd-4f2c-a2de-10609b032a8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_c3d177b7-165b-4976-b52e-e65b02364e91" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:to="loc_srt_RangeAxis_c3d177b7-165b-4976-b52e-e65b02364e91" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_c3d177b7-165b-4976-b52e-e65b02364e91_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_c3d177b7-165b-4976-b52e-e65b02364e91" xlink:to="loc_srt_RangeMember_c3d177b7-165b-4976-b52e-e65b02364e91_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_b944289c-6e71-4dc1-8b19-12fb6f0adff2" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_c3d177b7-165b-4976-b52e-e65b02364e91" xlink:to="loc_srt_RangeMember_b944289c-6e71-4dc1-8b19-12fb6f0adff2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_dd1c9155-37db-4f53-a2f1-1b9000c21fdc" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b944289c-6e71-4dc1-8b19-12fb6f0adff2" xlink:to="loc_srt_MinimumMember_dd1c9155-37db-4f53-a2f1-1b9000c21fdc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_97d3af74-a3e3-484c-9c6c-6b38c7f7a42e" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b944289c-6e71-4dc1-8b19-12fb6f0adff2" xlink:to="loc_srt_MaximumMember_97d3af74-a3e3-484c-9c6c-6b38c7f7a42e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_f7e31c2b-b093-43e8-81b3-6eb37ab71d2e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_f7e31c2b-b093-43e8-81b3-6eb37ab71d2e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_f7e31c2b-b093-43e8-81b3-6eb37ab71d2e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_f7e31c2b-b093-43e8-81b3-6eb37ab71d2e" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_f7e31c2b-b093-43e8-81b3-6eb37ab71d2e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_43caac34-dc92-4be4-b561-32404bff9312" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_f7e31c2b-b093-43e8-81b3-6eb37ab71d2e" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_43caac34-dc92-4be4-b561-32404bff9312" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeSeveranceMember_d461a43d-d7eb-4c6e-a632-40af1f1211f7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeSeveranceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_43caac34-dc92-4be4-b561-32404bff9312" xlink:to="loc_us-gaap_EmployeeSeveranceMember_d461a43d-d7eb-4c6e-a632-40af1f1211f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherRestructuringMember_5cf5058c-bd4c-4844-9f2b-4ced477613ba" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherRestructuringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_43caac34-dc92-4be4-b561-32404bff9312" xlink:to="loc_us-gaap_OtherRestructuringMember_5cf5058c-bd4c-4844-9f2b-4ced477613ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanAxis_feee46eb-393d-4367-a850-4b490747d303" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringPlanAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dco_SignificantAccountingPoliciesTable_cff9c2ec-1205-4245-bdba-eeba6d7f05e1" xlink:to="loc_us-gaap_RestructuringPlanAxis_feee46eb-393d-4367-a850-4b490747d303" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_feee46eb-393d-4367-a850-4b490747d303_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_feee46eb-393d-4367-a850-4b490747d303" xlink:to="loc_us-gaap_RestructuringPlanDomain_feee46eb-393d-4367-a850-4b490747d303_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringPlanDomain_0fe62f77-0a04-429b-8c12-0c1b170a4029" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestructuringPlanDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_feee46eb-393d-4367-a850-4b490747d303" xlink:to="loc_us-gaap_RestructuringPlanDomain_0fe62f77-0a04-429b-8c12-0c1b170a4029" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_April2022RestructuringPlanMember_5db20d37-02cf-4aed-a8ac-c30c12a205e6" xlink:href="dco-20220402.xsd#dco_April2022RestructuringPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_0fe62f77-0a04-429b-8c12-0c1b170a4029" xlink:to="loc_dco_April2022RestructuringPlanMember_5db20d37-02cf-4aed-a8ac-c30c12a205e6" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/SummaryofSignificantAccountingPoliciesWeightedAverageNumberofSharesOutstandingExcludedfromComputationofDilutedEarningsDetails" xlink:type="simple" xlink:href="dco-20220402.xsd#SummaryofSignificantAccountingPoliciesWeightedAverageNumberofSharesOutstandingExcludedfromComputationofDilutedEarningsDetails"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/SummaryofSignificantAccountingPoliciesWeightedAverageNumberofSharesOutstandingExcludedfromComputationofDilutedEarningsDetails" xlink:type="extended" id="i78658c8719964d448e5c33d55c98088e_SummaryofSignificantAccountingPoliciesWeightedAverageNumberofSharesOutstandingExcludedfromComputationofDilutedEarningsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_ae0b3ff2-3c22-4c54-a06f-e2f1611ce272" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_9546a645-4362-456f-98b7-52c73fb33617" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_ae0b3ff2-3c22-4c54-a06f-e2f1611ce272" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_9546a645-4362-456f-98b7-52c73fb33617" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_dcfb9e38-eeb9-49c5-b8cc-c355e41a83e1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_ae0b3ff2-3c22-4c54-a06f-e2f1611ce272" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_dcfb9e38-eeb9-49c5-b8cc-c355e41a83e1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_ca7b5010-123c-4e11-8ec2-074ed64ad145" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_dcfb9e38-eeb9-49c5-b8cc-c355e41a83e1" xlink:to="loc_us-gaap_AwardTypeAxis_ca7b5010-123c-4e11-8ec2-074ed64ad145" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ca7b5010-123c-4e11-8ec2-074ed64ad145_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_ca7b5010-123c-4e11-8ec2-074ed64ad145" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ca7b5010-123c-4e11-8ec2-074ed64ad145_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ba4039b3-cfe2-4a75-8bf1-300f3dc1dfa1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_ca7b5010-123c-4e11-8ec2-074ed64ad145" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ba4039b3-cfe2-4a75-8bf1-300f3dc1dfa1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_StockOptionsAndRestrictedStockUnitsMember_66c85bf8-83e9-4be5-8cc4-d410f0e1df53" xlink:href="dco-20220402.xsd#dco_StockOptionsAndRestrictedStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ba4039b3-cfe2-4a75-8bf1-300f3dc1dfa1" xlink:to="loc_dco_StockOptionsAndRestrictedStockUnitsMember_66c85bf8-83e9-4be5-8cc4-d410f0e1df53" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/SummaryofSignificantAccountingPoliciesDisaggregatedRevenueDetails" xlink:type="simple" xlink:href="dco-20220402.xsd#SummaryofSignificantAccountingPoliciesDisaggregatedRevenueDetails"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/SummaryofSignificantAccountingPoliciesDisaggregatedRevenueDetails" xlink:type="extended" id="i8e01dae394f64b5e9c6e0dcd90a19d84_SummaryofSignificantAccountingPoliciesDisaggregatedRevenueDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_a3184ea2-b3d7-430a-b493-e365a8fdf8ce" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_1e73a9a6-a14b-414d-9f6d-8c62b6574c06" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_a3184ea2-b3d7-430a-b493-e365a8fdf8ce" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_1e73a9a6-a14b-414d-9f6d-8c62b6574c06" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_5000a537-e30c-4f10-93ed-a03793628741" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_a3184ea2-b3d7-430a-b493-e365a8fdf8ce" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_5000a537-e30c-4f10-93ed-a03793628741" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_b60e0b1a-fef5-467e-9122-f62ff84b8fe7" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_5000a537-e30c-4f10-93ed-a03793628741" xlink:to="loc_srt_ProductOrServiceAxis_b60e0b1a-fef5-467e-9122-f62ff84b8fe7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_b60e0b1a-fef5-467e-9122-f62ff84b8fe7_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_b60e0b1a-fef5-467e-9122-f62ff84b8fe7" xlink:to="loc_srt_ProductsAndServicesDomain_b60e0b1a-fef5-467e-9122-f62ff84b8fe7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_7afd6ea4-ba6c-41fb-a92d-ced79466c828" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_b60e0b1a-fef5-467e-9122-f62ff84b8fe7" xlink:to="loc_srt_ProductsAndServicesDomain_7afd6ea4-ba6c-41fb-a92d-ced79466c828" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_MilitaryandSpaceMember_9f2257f5-d6ce-46db-88ea-cf312340fe9e" xlink:href="dco-20220402.xsd#dco_MilitaryandSpaceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_7afd6ea4-ba6c-41fb-a92d-ced79466c828" xlink:to="loc_dco_MilitaryandSpaceMember_9f2257f5-d6ce-46db-88ea-cf312340fe9e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_CommercialAerospaceMember_354dbfe4-8a5f-4ded-b3e2-4ec9669adf43" xlink:href="dco-20220402.xsd#dco_CommercialAerospaceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_7afd6ea4-ba6c-41fb-a92d-ced79466c828" xlink:to="loc_dco_CommercialAerospaceMember_354dbfe4-8a5f-4ded-b3e2-4ec9669adf43" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_IndustrialMember_2c95beed-155c-4bfa-a822-ea01ad8a3bae" xlink:href="dco-20220402.xsd#dco_IndustrialMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_7afd6ea4-ba6c-41fb-a92d-ced79466c828" xlink:to="loc_dco_IndustrialMember_2c95beed-155c-4bfa-a822-ea01ad8a3bae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_e066a1e5-121d-41cd-b3ff-0a8f5feb0cce" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_5000a537-e30c-4f10-93ed-a03793628741" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_e066a1e5-121d-41cd-b3ff-0a8f5feb0cce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_e066a1e5-121d-41cd-b3ff-0a8f5feb0cce_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_e066a1e5-121d-41cd-b3ff-0a8f5feb0cce" xlink:to="loc_us-gaap_SegmentDomain_e066a1e5-121d-41cd-b3ff-0a8f5feb0cce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_21f6a88e-3ede-4f0e-8a78-18f78ea6fa65" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_e066a1e5-121d-41cd-b3ff-0a8f5feb0cce" xlink:to="loc_us-gaap_SegmentDomain_21f6a88e-3ede-4f0e-8a78-18f78ea6fa65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_ElectronicSystemsMember_a0649824-3a9e-4cc7-9a05-b3da6596120b" xlink:href="dco-20220402.xsd#dco_ElectronicSystemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_21f6a88e-3ede-4f0e-8a78-18f78ea6fa65" xlink:to="loc_dco_ElectronicSystemsMember_a0649824-3a9e-4cc7-9a05-b3da6596120b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_StructuralSystemsMember_44a5c59f-eb4e-406f-9fe6-3e83cc1c4a9b" xlink:href="dco-20220402.xsd#dco_StructuralSystemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_21f6a88e-3ede-4f0e-8a78-18f78ea6fa65" xlink:to="loc_dco_StructuralSystemsMember_44a5c59f-eb4e-406f-9fe6-3e83cc1c4a9b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/BusinessCombinationsNarrativeDetails" xlink:type="simple" xlink:href="dco-20220402.xsd#BusinessCombinationsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/BusinessCombinationsNarrativeDetails" xlink:type="extended" id="id1682976d0af49cfaccbaa39efbbd87c_BusinessCombinationsNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_a1b3dab7-c047-42e4-b74b-83f2c4b89ee2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_f1809fdd-c917-4058-9fb3-e1805de2c416" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a1b3dab7-c047-42e4-b74b-83f2c4b89ee2" xlink:to="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_f1809fdd-c917-4058-9fb3-e1805de2c416" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_0e5eb315-4be6-4141-b43a-9751f38450b8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a1b3dab7-c047-42e4-b74b-83f2c4b89ee2" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_0e5eb315-4be6-4141-b43a-9751f38450b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_cbafc695-2bce-48c0-9718-b831ca9b1c35" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a1b3dab7-c047-42e4-b74b-83f2c4b89ee2" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_cbafc695-2bce-48c0-9718-b831ca9b1c35" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_d112bd52-e5a1-475a-a879-c18a0352f090" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a1b3dab7-c047-42e4-b74b-83f2c4b89ee2" xlink:to="loc_us-gaap_Goodwill_d112bd52-e5a1-475a-a879-c18a0352f090" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_94531434-9a5c-48d2-a85a-cd07c535b507" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a1b3dab7-c047-42e4-b74b-83f2c4b89ee2" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_94531434-9a5c-48d2-a85a-cd07c535b507" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_25bbb232-702b-4124-ab3b-e23390761574" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a1b3dab7-c047-42e4-b74b-83f2c4b89ee2" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_25bbb232-702b-4124-ab3b-e23390761574" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_c774e4ab-113a-486c-992d-e32ed94ffab0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_a1b3dab7-c047-42e4-b74b-83f2c4b89ee2" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_c774e4ab-113a-486c-992d-e32ed94ffab0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_0f2cb7e3-dad3-490c-9595-89a2ff24264c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_c774e4ab-113a-486c-992d-e32ed94ffab0" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_0f2cb7e3-dad3-490c-9595-89a2ff24264c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0f2cb7e3-dad3-490c-9595-89a2ff24264c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0f2cb7e3-dad3-490c-9595-89a2ff24264c" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0f2cb7e3-dad3-490c-9595-89a2ff24264c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_83009603-3652-4adc-8c24-83bd0f495714" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0f2cb7e3-dad3-490c-9595-89a2ff24264c" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_83009603-3652-4adc-8c24-83bd0f495714" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_MagneticSealCorporationMember_c4f2f1b0-7331-48d8-84ea-507a1e83056c" xlink:href="dco-20220402.xsd#dco_MagneticSealCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_83009603-3652-4adc-8c24-83bd0f495714" xlink:to="loc_dco_MagneticSealCorporationMember_c4f2f1b0-7331-48d8-84ea-507a1e83056c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/BusinessCombinationsFairValueofAssetsandLiabilitiesAcquiredDetails" xlink:type="simple" xlink:href="dco-20220402.xsd#BusinessCombinationsFairValueofAssetsandLiabilitiesAcquiredDetails"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/BusinessCombinationsFairValueofAssetsandLiabilitiesAcquiredDetails" xlink:type="extended" id="ia298249cf3594cad9187d0c33a681ac2_BusinessCombinationsFairValueofAssetsandLiabilitiesAcquiredDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_37a94baa-5d34-4057-9f55-e62bef55c862" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_37a94baa-5d34-4057-9f55-e62bef55c862" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_c869131a-5bba-4dfe-9100-cb3ebea78a22" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_c869131a-5bba-4dfe-9100-cb3ebea78a22" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory_8f49f92e-4fe1-4d44-98ee-c9dceb0a19a0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory_8f49f92e-4fe1-4d44-98ee-c9dceb0a19a0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther_4cfe806a-1848-4ca0-89e6-5b3cacd440ce" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther_4cfe806a-1848-4ca0-89e6-5b3cacd440ce" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_579f0ae5-9468-4914-aeb0-ec4c7ff64612" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_579f0ae5-9468-4914-aeb0-ec4c7ff64612" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAssets_91e12993-4a47-4738-a56a-c28d6e381d37" xlink:href="dco-20220402.xsd#dco_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_dco_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeaseRightOfUseAssets_91e12993-4a47-4738-a56a-c28d6e381d37" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_cfd1d36c-dff0-48a0-9ab6-35f8768557f3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_cfd1d36c-dff0-48a0-9ab6-35f8768557f3" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_525d54cf-038b-47e2-9c07-3ee5a0ee8536" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_us-gaap_Goodwill_525d54cf-038b-47e2-9c07-3ee5a0ee8536" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedAssets_dbbfc3e7-5c18-48d6-bf99-3f8ff0ce517c" xlink:href="dco-20220402.xsd#dco_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_dco_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedAssets_dbbfc3e7-5c18-48d6-bf99-3f8ff0ce517c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_57788dec-5cd9-444f-b973-90f9513a4707" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_57788dec-5cd9-444f-b973-90f9513a4707" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_ac96e1dd-14a4-4707-bae2-4ba581d3c296" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_ac96e1dd-14a4-4707-bae2-4ba581d3c296" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_cc72e3ef-a320-4137-a1da-be9e52c0d855" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_cc72e3ef-a320-4137-a1da-be9e52c0d855" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_6fe27185-9cff-438f-aa23-c0c33b9ad594" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_6fe27185-9cff-438f-aa23-c0c33b9ad594" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_9dbb261b-957b-477e-89cc-3fe0cd036117" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_59d97efd-d710-4a08-a657-e3f417352b89" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_9dbb261b-957b-477e-89cc-3fe0cd036117" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_9d17ff24-4322-47e8-8bc0-719f2d309656" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_9dbb261b-957b-477e-89cc-3fe0cd036117" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_9d17ff24-4322-47e8-8bc0-719f2d309656" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9d17ff24-4322-47e8-8bc0-719f2d309656_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_9d17ff24-4322-47e8-8bc0-719f2d309656" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9d17ff24-4322-47e8-8bc0-719f2d309656_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5cad2a61-3f78-447d-bb8e-f310a9869180" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_9d17ff24-4322-47e8-8bc0-719f2d309656" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5cad2a61-3f78-447d-bb8e-f310a9869180" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_MagneticSealCorporationMember_d08dd0f3-407d-49c9-8965-6e6e4f262bf1" xlink:href="dco-20220402.xsd#dco_MagneticSealCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5cad2a61-3f78-447d-bb8e-f310a9869180" xlink:to="loc_dco_MagneticSealCorporationMember_d08dd0f3-407d-49c9-8965-6e6e4f262bf1" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/BusinessCombinationsEstimatedFairValueofIntangibleAssetsAcquiredDetails" xlink:type="simple" xlink:href="dco-20220402.xsd#BusinessCombinationsEstimatedFairValueofIntangibleAssetsAcquiredDetails"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/BusinessCombinationsEstimatedFairValueofIntangibleAssetsAcquiredDetails" xlink:type="extended" id="idb21f187bae24ab0a6021470e0a6c73f_BusinessCombinationsEstimatedFairValueofIntangibleAssetsAcquiredDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_22f5b2a1-e7ab-49e5-8a16-bb1bd433ff56" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_79669508-64b5-4455-9282-d96237e96463" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_22f5b2a1-e7ab-49e5-8a16-bb1bd433ff56" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_79669508-64b5-4455-9282-d96237e96463" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_6066fbfc-4f47-476d-a848-e7bfc8ee19a3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_22f5b2a1-e7ab-49e5-8a16-bb1bd433ff56" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_6066fbfc-4f47-476d-a848-e7bfc8ee19a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets_b34f4a6e-4643-4418-8e9f-86f48d0b21eb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_22f5b2a1-e7ab-49e5-8a16-bb1bd433ff56" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets_b34f4a6e-4643-4418-8e9f-86f48d0b21eb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_aba44235-d7a6-4a9d-ad63-bdf9224ecadf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_22f5b2a1-e7ab-49e5-8a16-bb1bd433ff56" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_aba44235-d7a6-4a9d-ad63-bdf9224ecadf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_e0083fb1-a2d3-4635-902f-8cc3e2a26921" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_22f5b2a1-e7ab-49e5-8a16-bb1bd433ff56" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_e0083fb1-a2d3-4635-902f-8cc3e2a26921" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_95dc8d95-2ea4-4f2e-b7d4-4b3a09f0feda" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_e0083fb1-a2d3-4635-902f-8cc3e2a26921" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_95dc8d95-2ea4-4f2e-b7d4-4b3a09f0feda" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_95dc8d95-2ea4-4f2e-b7d4-4b3a09f0feda_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_95dc8d95-2ea4-4f2e-b7d4-4b3a09f0feda" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_95dc8d95-2ea4-4f2e-b7d4-4b3a09f0feda_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e01c4727-f0e9-4f56-b805-378bc6871fd5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_95dc8d95-2ea4-4f2e-b7d4-4b3a09f0feda" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e01c4727-f0e9-4f56-b805-378bc6871fd5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_MagneticSealCorporationMember_dbba4402-c431-4cff-a043-072ea26d46a0" xlink:href="dco-20220402.xsd#dco_MagneticSealCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_e01c4727-f0e9-4f56-b805-378bc6871fd5" xlink:to="loc_dco_MagneticSealCorporationMember_dbba4402-c431-4cff-a043-072ea26d46a0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_a6952904-82e3-4d1e-b73e-f1d7e65db083" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_e0083fb1-a2d3-4635-902f-8cc3e2a26921" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_a6952904-82e3-4d1e-b73e-f1d7e65db083" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_a6952904-82e3-4d1e-b73e-f1d7e65db083_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_a6952904-82e3-4d1e-b73e-f1d7e65db083" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_a6952904-82e3-4d1e-b73e-f1d7e65db083_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_12086def-5148-4a94-9f7a-8927a1453add" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_a6952904-82e3-4d1e-b73e-f1d7e65db083" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_12086def-5148-4a94-9f7a-8927a1453add" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_c6370e37-291a-4447-a5cc-1247c969cbbe" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_12086def-5148-4a94-9f7a-8927a1453add" xlink:to="loc_us-gaap_CustomerRelationshipsMember_c6370e37-291a-4447-a5cc-1247c969cbbe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OrderOrProductionBacklogMember_0432ef43-580d-4fd5-8421-742bf88f0567" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OrderOrProductionBacklogMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_12086def-5148-4a94-9f7a-8927a1453add" xlink:to="loc_us-gaap_OrderOrProductionBacklogMember_0432ef43-580d-4fd5-8421-742bf88f0567" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_6aadb478-4c9e-441d-b0dc-5b99601b86c8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_e0083fb1-a2d3-4635-902f-8cc3e2a26921" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_6aadb478-4c9e-441d-b0dc-5b99601b86c8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_6aadb478-4c9e-441d-b0dc-5b99601b86c8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_6aadb478-4c9e-441d-b0dc-5b99601b86c8" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_6aadb478-4c9e-441d-b0dc-5b99601b86c8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_3b01a150-2f72-4b5d-b5b2-cd3c6e3b1389" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_6aadb478-4c9e-441d-b0dc-5b99601b86c8" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_3b01a150-2f72-4b5d-b5b2-cd3c6e3b1389" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_e19aa065-686c-4119-8062-1b79473aa008" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_3b01a150-2f72-4b5d-b5b2-cd3c6e3b1389" xlink:to="loc_us-gaap_TradeNamesMember_e19aa065-686c-4119-8062-1b79473aa008" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/GoodwillNarrativeDetails" xlink:type="simple" xlink:href="dco-20220402.xsd#GoodwillNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/GoodwillNarrativeDetails" xlink:type="extended" id="ifca49711dcb844b9a03c9ef97fa3b6d8_GoodwillNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_8aa88762-1276-497c-b9de-bcb364535752" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount_296a47a9-b5cc-4b94-8c61-b7150123f07c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_8aa88762-1276-497c-b9de-bcb364535752" xlink:to="loc_us-gaap_ReportingUnitPercentageOfFairValueInExcessOfCarryingAmount_296a47a9-b5cc-4b94-8c61-b7150123f07c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_8430933e-7ff6-4ffc-95a8-88c08e71e233" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_8aa88762-1276-497c-b9de-bcb364535752" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_8430933e-7ff6-4ffc-95a8-88c08e71e233" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_31c5c32c-4b82-4426-ac3e-e60aa437169f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_8430933e-7ff6-4ffc-95a8-88c08e71e233" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_31c5c32c-4b82-4426-ac3e-e60aa437169f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_31c5c32c-4b82-4426-ac3e-e60aa437169f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_31c5c32c-4b82-4426-ac3e-e60aa437169f" xlink:to="loc_us-gaap_SegmentDomain_31c5c32c-4b82-4426-ac3e-e60aa437169f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_a5bf1946-2294-45a5-9d04-6df47859cf1a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_31c5c32c-4b82-4426-ac3e-e60aa437169f" xlink:to="loc_us-gaap_SegmentDomain_a5bf1946-2294-45a5-9d04-6df47859cf1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_StructuralSystemsMember_37ccdcc9-da75-4c1f-a6b3-8229813e4aad" xlink:href="dco-20220402.xsd#dco_StructuralSystemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_a5bf1946-2294-45a5-9d04-6df47859cf1a" xlink:to="loc_dco_StructuralSystemsMember_37ccdcc9-da75-4c1f-a6b3-8229813e4aad" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/GoodwillDetails" xlink:type="simple" xlink:href="dco-20220402.xsd#GoodwillDetails"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/GoodwillDetails" xlink:type="extended" id="ia33fa4ccbcb442ec9fe3f0637e5664b5_GoodwillDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_e45fa7b5-7b7a-42c2-a0d9-b4d167293c89" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillGross_7e6df2a9-30d0-4188-93be-18bae2c70118" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_e45fa7b5-7b7a-42c2-a0d9-b4d167293c89" xlink:to="loc_us-gaap_GoodwillGross_7e6df2a9-30d0-4188-93be-18bae2c70118" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillImpairedAccumulatedImpairmentLoss_7eb35374-35db-4d37-bee3-f227f1c288c9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillImpairedAccumulatedImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_e45fa7b5-7b7a-42c2-a0d9-b4d167293c89" xlink:to="loc_us-gaap_GoodwillImpairedAccumulatedImpairmentLoss_7eb35374-35db-4d37-bee3-f227f1c288c9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillRollForward_298f153d-c16d-4cdb-9336-f5fad3f913a2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_e45fa7b5-7b7a-42c2-a0d9-b4d167293c89" xlink:to="loc_us-gaap_GoodwillRollForward_298f153d-c16d-4cdb-9336-f5fad3f913a2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_7d758519-f881-467f-b8ca-0007e55dded8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_298f153d-c16d-4cdb-9336-f5fad3f913a2" xlink:to="loc_us-gaap_Goodwill_7d758519-f881-467f-b8ca-0007e55dded8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_10cf15fd-b797-40fa-8c48-6c3e575c8967" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_e45fa7b5-7b7a-42c2-a0d9-b4d167293c89" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_10cf15fd-b797-40fa-8c48-6c3e575c8967" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_f0a2269e-cb76-4c76-a17e-9bbf4bf46a2e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_10cf15fd-b797-40fa-8c48-6c3e575c8967" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_f0a2269e-cb76-4c76-a17e-9bbf4bf46a2e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_f0a2269e-cb76-4c76-a17e-9bbf4bf46a2e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_f0a2269e-cb76-4c76-a17e-9bbf4bf46a2e" xlink:to="loc_us-gaap_SegmentDomain_f0a2269e-cb76-4c76-a17e-9bbf4bf46a2e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_383c5bcf-962e-4bfe-b158-03f9dfb1eca9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_f0a2269e-cb76-4c76-a17e-9bbf4bf46a2e" xlink:to="loc_us-gaap_SegmentDomain_383c5bcf-962e-4bfe-b158-03f9dfb1eca9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_ElectronicSystemsMember_be4b9680-2603-4c77-99cf-f79de7bcd87b" xlink:href="dco-20220402.xsd#dco_ElectronicSystemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_383c5bcf-962e-4bfe-b158-03f9dfb1eca9" xlink:to="loc_dco_ElectronicSystemsMember_be4b9680-2603-4c77-99cf-f79de7bcd87b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_StructuralSystemsMember_eb962675-e304-4980-8939-86557c51393b" xlink:href="dco-20220402.xsd#dco_StructuralSystemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_383c5bcf-962e-4bfe-b158-03f9dfb1eca9" xlink:to="loc_dco_StructuralSystemsMember_eb962675-e304-4980-8939-86557c51393b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/LongTermDebtSummaryDetails" xlink:type="simple" xlink:href="dco-20220402.xsd#LongTermDebtSummaryDetails"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/LongTermDebtSummaryDetails" xlink:type="extended" id="iaf32cd1aabcc49c49c2e65c3c61ff9c8_LongTermDebtSummaryDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_c6f44271-0e65-4379-aaa1-4a9a132cde7a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_58bcae43-8261-4c09-8116-ad8b53a8eec5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_c6f44271-0e65-4379-aaa1-4a9a132cde7a" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_58bcae43-8261-4c09-8116-ad8b53a8eec5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_c74145b4-c17c-435b-ab58-b70ddeccc886" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_c6f44271-0e65-4379-aaa1-4a9a132cde7a" xlink:to="loc_us-gaap_LongTermDebtCurrent_c74145b4-c17c-435b-ab58-b70ddeccc886" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_b0eda0ac-214f-4e7b-811b-8dae86522b39" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_c6f44271-0e65-4379-aaa1-4a9a132cde7a" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_b0eda0ac-214f-4e7b-811b-8dae86522b39" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_84f3c314-8b5b-4d28-8dec-7bd4f33e3e00" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_c6f44271-0e65-4379-aaa1-4a9a132cde7a" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_84f3c314-8b5b-4d28-8dec-7bd4f33e3e00" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_0054310f-f9ba-477a-82ad-5345a589e539" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_c6f44271-0e65-4379-aaa1-4a9a132cde7a" xlink:to="loc_us-gaap_LongTermDebt_0054310f-f9ba-477a-82ad-5345a589e539" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtWeightedAverageInterestRate_7f875a81-8fc9-4f27-a462-0f7fb6b0258a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_c6f44271-0e65-4379-aaa1-4a9a132cde7a" xlink:to="loc_us-gaap_LongtermDebtWeightedAverageInterestRate_7f875a81-8fc9-4f27-a462-0f7fb6b0258a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_937f1386-1c5f-4bb2-a481-825f16daf5ff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_c6f44271-0e65-4379-aaa1-4a9a132cde7a" xlink:to="loc_us-gaap_DebtInstrumentTable_937f1386-1c5f-4bb2-a481-825f16daf5ff" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_93e80b87-ae58-44e5-aeb6-f53d3d3e8872" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_937f1386-1c5f-4bb2-a481-825f16daf5ff" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_93e80b87-ae58-44e5-aeb6-f53d3d3e8872" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_93e80b87-ae58-44e5-aeb6-f53d3d3e8872_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_93e80b87-ae58-44e5-aeb6-f53d3d3e8872" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_93e80b87-ae58-44e5-aeb6-f53d3d3e8872_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_8d325fa7-327c-48ca-a1fb-43660947c3ed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_93e80b87-ae58-44e5-aeb6-f53d3d3e8872" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_8d325fa7-327c-48ca-a1fb-43660947c3ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_SeniorSecuredTermLoanMember_888d9847-a277-4748-9eee-1b700c17bb45" xlink:href="dco-20220402.xsd#dco_SeniorSecuredTermLoanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_8d325fa7-327c-48ca-a1fb-43660947c3ed" xlink:to="loc_dco_SeniorSecuredTermLoanMember_888d9847-a277-4748-9eee-1b700c17bb45" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_33241435-a11d-4c62-8e95-dbe68f3507ae" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_8d325fa7-327c-48ca-a1fb-43660947c3ed" xlink:to="loc_us-gaap_LineOfCreditMember_33241435-a11d-4c62-8e95-dbe68f3507ae" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/LongTermDebtNarrativeDetails" xlink:type="simple" xlink:href="dco-20220402.xsd#LongTermDebtNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/LongTermDebtNarrativeDetails" xlink:type="extended" id="i4b1847c22f344c12b285045975488f6d_LongTermDebtNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_13903b0a-eaa5-44f5-bddb-bf9cf9ba7cde" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_13903b0a-eaa5-44f5-bddb-bf9cf9ba7cde" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_5b9ee885-d526-4aaf-99ad-ed7ec7c34609" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_5b9ee885-d526-4aaf-99ad-ed7ec7c34609" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_a2bf7cfc-d6c7-4f6b-b283-7b794d10cd73" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_a2bf7cfc-d6c7-4f6b-b283-7b794d10cd73" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_DebtInstrumentBasisSpreadonFixedRateBaseRate_d8d08cec-c49d-4de9-8f02-e6d4a8e633b1" xlink:href="dco-20220402.xsd#dco_DebtInstrumentBasisSpreadonFixedRateBaseRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_dco_DebtInstrumentBasisSpreadonFixedRateBaseRate_d8d08cec-c49d-4de9-8f02-e6d4a8e633b1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_DebtInstrumentPrepaymentAmountPercentageofPrincipalAmount_f74956a9-c6af-42a3-8080-c94140b0ba59" xlink:href="dco-20220402.xsd#dco_DebtInstrumentPrepaymentAmountPercentageofPrincipalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_dco_DebtInstrumentPrepaymentAmountPercentageofPrincipalAmount_f74956a9-c6af-42a3-8080-c94140b0ba59" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfDebt_7cecb04c-f4f5-4ab8-bc81-bcdc0b762550" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RepaymentsOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_RepaymentsOfDebt_7cecb04c-f4f5-4ab8-bc81-bcdc0b762550" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityCommitmentFeePercentage_1c656842-e45b-420b-8b18-3064a4fe42a3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditFacilityCommitmentFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_LineOfCreditFacilityCommitmentFeePercentage_1c656842-e45b-420b-8b18-3064a4fe42a3" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_DebtInstrumentPercentageofExcessCashFlowPaymentWhenLeverageRatioisGreaterthan3.25_6fb8753c-e825-441e-8dcc-47b4a7d11222" xlink:href="dco-20220402.xsd#dco_DebtInstrumentPercentageofExcessCashFlowPaymentWhenLeverageRatioisGreaterthan3.25"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_dco_DebtInstrumentPercentageofExcessCashFlowPaymentWhenLeverageRatioisGreaterthan3.25_6fb8753c-e825-441e-8dcc-47b4a7d11222" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_DebtInstrumentPercentageOfExcessCashFlowPaymentWhenLeverageRatioIsLessThanOrEqualTo325ButGreaterThan250_54dcebbe-edd6-46e9-8326-70ccdfb0d97a" xlink:href="dco-20220402.xsd#dco_DebtInstrumentPercentageOfExcessCashFlowPaymentWhenLeverageRatioIsLessThanOrEqualTo325ButGreaterThan250"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_dco_DebtInstrumentPercentageOfExcessCashFlowPaymentWhenLeverageRatioIsLessThanOrEqualTo325ButGreaterThan250_54dcebbe-edd6-46e9-8326-70ccdfb0d97a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_DebtInstrumentPercentageofExcessCashFlowPaymentWhenLeverageRatioisLessThanorEqualto2.50_9f90355a-b49e-4ab0-a699-3141e3dd527c" xlink:href="dco-20220402.xsd#dco_DebtInstrumentPercentageofExcessCashFlowPaymentWhenLeverageRatioisLessThanorEqualto2.50"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_dco_DebtInstrumentPercentageofExcessCashFlowPaymentWhenLeverageRatioisLessThanorEqualto2.50_9f90355a-b49e-4ab0-a699-3141e3dd527c" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromLinesOfCredit_7f449c27-5bf6-4c0e-a809-54703a17b4d5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProceedsFromLinesOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_ProceedsFromLinesOfCredit_7f449c27-5bf6-4c0e-a809-54703a17b4d5" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLinesOfCredit_c8c3151f-653d-456f-a889-4c4edb95c78d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RepaymentsOfLinesOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_RepaymentsOfLinesOfCredit_c8c3151f-653d-456f-a889-4c4edb95c78d" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsGross_f8f9a6ab-8fe7-4420-9f6c-ffa1fccd8a08" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredFinanceCostsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_DeferredFinanceCostsGross_f8f9a6ab-8fe7-4420-9f6c-ffa1fccd8a08" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_ae877384-07bc-4006-838d-750c629f42e3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_ae877384-07bc-4006-838d-750c629f42e3" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet_8b196198-f543-4e3f-8c24-600ad8ebe219" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_DebtIssuanceCostsLineOfCreditArrangementsNet_8b196198-f543-4e3f-8c24-600ad8ebe219" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_d2953639-8d81-49e6-9bc8-fe397c67e335" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_d2953639-8d81-49e6-9bc8-fe397c67e335" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_05927a88-75a1-4920-a01f-a7a4ba887c42" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_05927a88-75a1-4920-a01f-a7a4ba887c42" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_NumberofSubsidiariesThatAreNotGuarantorsonDebt_bb785f8d-ad44-4bf1-8514-3a0fd8dc03d6" xlink:href="dco-20220402.xsd#dco_NumberofSubsidiariesThatAreNotGuarantorsonDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_dco_NumberofSubsidiariesThatAreNotGuarantorsonDebt_bb785f8d-ad44-4bf1-8514-3a0fd8dc03d6" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_4f862b28-f508-4213-a56a-294132ac445d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_DerivativeNotionalAmount_4f862b28-f508-4213-a56a-294132ac445d" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAverageFixedInterestRate_35970e38-9227-4b50-bb25-d105e1eb9883" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DerivativeAverageFixedInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_DerivativeAverageFixedInterestRate_35970e38-9227-4b50-bb25-d105e1eb9883" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax_02d7390b-a53b-4fae-81fa-d9256d498e75" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax_02d7390b-a53b-4fae-81fa-d9256d498e75" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_ee309532-865b-426e-88aa-cc8679ccffd3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_91ac1b83-eb51-45d3-b076-6c674fb6c32f" xlink:to="loc_us-gaap_DebtInstrumentTable_ee309532-865b-426e-88aa-cc8679ccffd3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_7a75a71b-28e0-4c87-99de-344405053e14" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_ee309532-865b-426e-88aa-cc8679ccffd3" xlink:to="loc_us-gaap_CreditFacilityAxis_7a75a71b-28e0-4c87-99de-344405053e14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_7a75a71b-28e0-4c87-99de-344405053e14_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_7a75a71b-28e0-4c87-99de-344405053e14" xlink:to="loc_us-gaap_CreditFacilityDomain_7a75a71b-28e0-4c87-99de-344405053e14_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_df6df94c-6e8a-4973-adc9-ecb64f44d605" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_7a75a71b-28e0-4c87-99de-344405053e14" xlink:to="loc_us-gaap_CreditFacilityDomain_df6df94c-6e8a-4973-adc9-ecb64f44d605" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_a33cd809-ce81-4a73-ad96-a7eea822278a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_df6df94c-6e8a-4973-adc9-ecb64f44d605" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_a33cd809-ce81-4a73-ad96-a7eea822278a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_58242ffc-e34c-4c1d-98dc-a7bcca3bfe96" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_df6df94c-6e8a-4973-adc9-ecb64f44d605" xlink:to="loc_us-gaap_SecuredDebtMember_58242ffc-e34c-4c1d-98dc-a7bcca3bfe96" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_d406583a-7640-4f56-b476-f3e3275b3419" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_ee309532-865b-426e-88aa-cc8679ccffd3" xlink:to="loc_us-gaap_DebtInstrumentAxis_d406583a-7640-4f56-b476-f3e3275b3419" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_d406583a-7640-4f56-b476-f3e3275b3419_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_d406583a-7640-4f56-b476-f3e3275b3419" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_d406583a-7640-4f56-b476-f3e3275b3419_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_35a9825f-d857-4b51-b51b-3f37ee4e0dcf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_d406583a-7640-4f56-b476-f3e3275b3419" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_35a9825f-d857-4b51-b51b-3f37ee4e0dcf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_A2019RevolvingCreditFacilityDueDecember202024Member_8d660e9b-ef9a-4418-925d-9966732d15f2" xlink:href="dco-20220402.xsd#dco_A2019RevolvingCreditFacilityDueDecember202024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_35a9825f-d857-4b51-b51b-3f37ee4e0dcf" xlink:to="loc_dco_A2019RevolvingCreditFacilityDueDecember202024Member_8d660e9b-ef9a-4418-925d-9966732d15f2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_New2018RevolvingCreditFacilityMaturingNovember212023Member_48d4d514-2a15-4704-a5dd-6dacddb5bab9" xlink:href="dco-20220402.xsd#dco_New2018RevolvingCreditFacilityMaturingNovember212023Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_35a9825f-d857-4b51-b51b-3f37ee4e0dcf" xlink:to="loc_dco_New2018RevolvingCreditFacilityMaturingNovember212023Member_48d4d514-2a15-4704-a5dd-6dacddb5bab9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_A2019TermLoanMaturingDecember202024Member_a6f27937-28ca-43e2-af8c-120fcc80ecb6" xlink:href="dco-20220402.xsd#dco_A2019TermLoanMaturingDecember202024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_35a9825f-d857-4b51-b51b-3f37ee4e0dcf" xlink:to="loc_dco_A2019TermLoanMaturingDecember202024Member_a6f27937-28ca-43e2-af8c-120fcc80ecb6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_New2018TermLoanMaturingNovember212025Member_185d0dac-42cd-4741-89c2-708cf92a1992" xlink:href="dco-20220402.xsd#dco_New2018TermLoanMaturingNovember212025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_35a9825f-d857-4b51-b51b-3f37ee4e0dcf" xlink:to="loc_dco_New2018TermLoanMaturingNovember212025Member_185d0dac-42cd-4741-89c2-708cf92a1992" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_CreditFacilitiesMember_ed2e3cf0-556c-492e-8e57-97183d0dcfb8" xlink:href="dco-20220402.xsd#dco_CreditFacilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_35a9825f-d857-4b51-b51b-3f37ee4e0dcf" xlink:to="loc_dco_CreditFacilitiesMember_ed2e3cf0-556c-492e-8e57-97183d0dcfb8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_NewTermLoanMaturingDecember202024Member_a83f5cf8-e16a-41cf-bcaa-a6cef7dbb702" xlink:href="dco-20220402.xsd#dco_NewTermLoanMaturingDecember202024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_35a9825f-d857-4b51-b51b-3f37ee4e0dcf" xlink:to="loc_dco_NewTermLoanMaturingDecember202024Member_a83f5cf8-e16a-41cf-bcaa-a6cef7dbb702" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_NewCreditFacilitiesMember_c8117b4f-6bb3-4284-afc8-4c5115efbe0f" xlink:href="dco-20220402.xsd#dco_NewCreditFacilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_35a9825f-d857-4b51-b51b-3f37ee4e0dcf" xlink:to="loc_dco_NewCreditFacilitiesMember_c8117b4f-6bb3-4284-afc8-4c5115efbe0f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_d6dac226-849e-4a99-a615-87405419d680" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_ee309532-865b-426e-88aa-cc8679ccffd3" xlink:to="loc_us-gaap_VariableRateAxis_d6dac226-849e-4a99-a615-87405419d680" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_d6dac226-849e-4a99-a615-87405419d680_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_d6dac226-849e-4a99-a615-87405419d680" xlink:to="loc_us-gaap_VariableRateDomain_d6dac226-849e-4a99-a615-87405419d680_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_f85d846d-76ca-4fc1-8b3f-a6c7b7b0807b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_d6dac226-849e-4a99-a615-87405419d680" xlink:to="loc_us-gaap_VariableRateDomain_f85d846d-76ca-4fc1-8b3f-a6c7b7b0807b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_3f326b12-a3e8-4903-b342-db1904cd7581" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LondonInterbankOfferedRateLIBORMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_f85d846d-76ca-4fc1-8b3f-a6c7b7b0807b" xlink:to="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_3f326b12-a3e8-4903-b342-db1904cd7581" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FederalFundsEffectiveSwapRateMember_833513ef-462c-4a01-833a-4dbca9ce3863" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FederalFundsEffectiveSwapRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_f85d846d-76ca-4fc1-8b3f-a6c7b7b0807b" xlink:to="loc_us-gaap_FederalFundsEffectiveSwapRateMember_833513ef-462c-4a01-833a-4dbca9ce3863" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EurodollarMember_4af72ffc-767f-43e2-8982-44bc5d70cbf3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EurodollarMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_f85d846d-76ca-4fc1-8b3f-a6c7b7b0807b" xlink:to="loc_us-gaap_EurodollarMember_4af72ffc-767f-43e2-8982-44bc5d70cbf3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BaseRateMember_19007d11-f1b8-406c-b9cf-8c02352f52ed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_f85d846d-76ca-4fc1-8b3f-a6c7b7b0807b" xlink:to="loc_us-gaap_BaseRateMember_19007d11-f1b8-406c-b9cf-8c02352f52ed" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_1209ae84-a3d9-4616-8cb6-687e6235e5fa" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_ee309532-865b-426e-88aa-cc8679ccffd3" xlink:to="loc_srt_RangeAxis_1209ae84-a3d9-4616-8cb6-687e6235e5fa" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_1209ae84-a3d9-4616-8cb6-687e6235e5fa_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_1209ae84-a3d9-4616-8cb6-687e6235e5fa" xlink:to="loc_srt_RangeMember_1209ae84-a3d9-4616-8cb6-687e6235e5fa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_b0bc1970-5974-45a8-b178-eb91e03bb969" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_1209ae84-a3d9-4616-8cb6-687e6235e5fa" xlink:to="loc_srt_RangeMember_b0bc1970-5974-45a8-b178-eb91e03bb969" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_1f37155f-54a4-482d-a73d-dfa8d2655ef9" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b0bc1970-5974-45a8-b178-eb91e03bb969" xlink:to="loc_srt_MinimumMember_1f37155f-54a4-482d-a73d-dfa8d2655ef9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_19947e00-a3a5-4087-a351-c1e16b35eddc" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_b0bc1970-5974-45a8-b178-eb91e03bb969" xlink:to="loc_srt_MaximumMember_19947e00-a3a5-4087-a351-c1e16b35eddc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_bb7faffd-6ee7-4af5-a325-0aa3a26a7218" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_ee309532-865b-426e-88aa-cc8679ccffd3" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_bb7faffd-6ee7-4af5-a325-0aa3a26a7218" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_bb7faffd-6ee7-4af5-a325-0aa3a26a7218_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_bb7faffd-6ee7-4af5-a325-0aa3a26a7218" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_bb7faffd-6ee7-4af5-a325-0aa3a26a7218_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_2018335d-8dbd-4a63-9265-ee2c5a536671" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_bb7faffd-6ee7-4af5-a325-0aa3a26a7218" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_2018335d-8dbd-4a63-9265-ee2c5a536671" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_bccd4e85-21ad-44ce-a6c9-9229e628d26f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_2018335d-8dbd-4a63-9265-ee2c5a536671" xlink:to="loc_us-gaap_InterestRateSwapMember_bccd4e85-21ad-44ce-a6c9-9229e628d26f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_7d9c84a9-2db8-437b-83e1-0fbed1e692ce" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_ee309532-865b-426e-88aa-cc8679ccffd3" xlink:to="loc_us-gaap_HedgingDesignationAxis_7d9c84a9-2db8-437b-83e1-0fbed1e692ce" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_7d9c84a9-2db8-437b-83e1-0fbed1e692ce_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_7d9c84a9-2db8-437b-83e1-0fbed1e692ce" xlink:to="loc_us-gaap_HedgingDesignationDomain_7d9c84a9-2db8-437b-83e1-0fbed1e692ce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_9c53ade1-5238-48c1-8fd1-911c645a7030" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_7d9c84a9-2db8-437b-83e1-0fbed1e692ce" xlink:to="loc_us-gaap_HedgingDesignationDomain_9c53ade1-5238-48c1-8fd1-911c645a7030" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_76c0461a-f99e-4315-b1ec-cd51fca07172" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_9c53ade1-5238-48c1-8fd1-911c645a7030" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_76c0461a-f99e-4315-b1ec-cd51fca07172" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/CommitmentsandContingenciesDetails" xlink:type="simple" xlink:href="dco-20220402.xsd#CommitmentsandContingenciesDetails"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/CommitmentsandContingenciesDetails" xlink:type="extended" id="i93248881ed274c1093ef7b13bd8a9121_CommitmentsandContingenciesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_163d75f7-0ebe-4c3b-8c4e-f8f0b02991a8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationReserve_af64e464-9690-4395-8f31-bad91e8ca429" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LitigationReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_163d75f7-0ebe-4c3b-8c4e-f8f0b02991a8" xlink:to="loc_us-gaap_LitigationReserve_af64e464-9690-4395-8f31-bad91e8ca429" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyAccrualAtCarryingValue_32c12cb2-3e95-47bb-99a9-ff104f4096e8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LossContingencyAccrualAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_163d75f7-0ebe-4c3b-8c4e-f8f0b02991a8" xlink:to="loc_us-gaap_LossContingencyAccrualAtCarryingValue_32c12cb2-3e95-47bb-99a9-ff104f4096e8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyEstimateOfPossibleLoss_835d34a5-ded7-4f9f-b623-e98f73a9b465" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LossContingencyEstimateOfPossibleLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_163d75f7-0ebe-4c3b-8c4e-f8f0b02991a8" xlink:to="loc_us-gaap_LossContingencyEstimateOfPossibleLoss_835d34a5-ded7-4f9f-b623-e98f73a9b465" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_LossContingencyNumberOfBuildingsAffected_d5f644c5-281a-4eaf-b4fb-431fa9c74417" xlink:href="dco-20220402.xsd#dco_LossContingencyNumberOfBuildingsAffected"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_163d75f7-0ebe-4c3b-8c4e-f8f0b02991a8" xlink:to="loc_dco_LossContingencyNumberOfBuildingsAffected_d5f644c5-281a-4eaf-b4fb-431fa9c74417" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AreaOfRealEstateProperty_40ea2f3e-3603-46cd-bc12-7c220ac86be4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AreaOfRealEstateProperty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_163d75f7-0ebe-4c3b-8c4e-f8f0b02991a8" xlink:to="loc_us-gaap_AreaOfRealEstateProperty_40ea2f3e-3603-46cd-bc12-7c220ac86be4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_4682916e-4bf1-4fb8-b2a6-412a31c369ef" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_163d75f7-0ebe-4c3b-8c4e-f8f0b02991a8" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_4682916e-4bf1-4fb8-b2a6-412a31c369ef" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_24bd3037-e977-4401-9282-04715e177cba" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_163d75f7-0ebe-4c3b-8c4e-f8f0b02991a8" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_24bd3037-e977-4401-9282-04715e177cba" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleCarryingValueOfAsset_138e0a6d-f4b2-437a-8963-f06ae75c892c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleCarryingValueOfAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_163d75f7-0ebe-4c3b-8c4e-f8f0b02991a8" xlink:to="loc_us-gaap_ImpairedAssetsToBeDisposedOfByMethodOtherThanSaleCarryingValueOfAsset_138e0a6d-f4b2-437a-8963-f06ae75c892c" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyReceivableProceeds_fc8af1a0-c5bb-49bd-b385-712333ec80c9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LossContingencyReceivableProceeds"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_163d75f7-0ebe-4c3b-8c4e-f8f0b02991a8" xlink:to="loc_us-gaap_LossContingencyReceivableProceeds_fc8af1a0-c5bb-49bd-b385-712333ec80c9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_a1353ab3-1b2b-4913-aef1-bfaec6242376" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_163d75f7-0ebe-4c3b-8c4e-f8f0b02991a8" xlink:to="loc_us-gaap_LossContingenciesTable_a1353ab3-1b2b-4913-aef1-bfaec6242376" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_30754cbe-9a7d-4c67-8bd2-7c238e49a1dc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_a1353ab3-1b2b-4913-aef1-bfaec6242376" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_30754cbe-9a7d-4c67-8bd2-7c238e49a1dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_30754cbe-9a7d-4c67-8bd2-7c238e49a1dc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_30754cbe-9a7d-4c67-8bd2-7c238e49a1dc" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_30754cbe-9a7d-4c67-8bd2-7c238e49a1dc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_7f5cd573-ceed-44e3-b722-199855bc8072" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_30754cbe-9a7d-4c67-8bd2-7c238e49a1dc" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_7f5cd573-ceed-44e3-b722-199855bc8072" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesMember_3ca2742f-ddd9-430a-bc69-03cd0a068115" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccruedLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_7f5cd573-ceed-44e3-b722-199855bc8072" xlink:to="loc_us-gaap_AccruedLiabilitiesMember_3ca2742f-ddd9-430a-bc69-03cd0a068115" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_098372e5-eb22-4b8c-a952-0567c5846987" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_a1353ab3-1b2b-4913-aef1-bfaec6242376" xlink:to="loc_srt_StatementGeographicalAxis_098372e5-eb22-4b8c-a952-0567c5846987" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_098372e5-eb22-4b8c-a952-0567c5846987_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_098372e5-eb22-4b8c-a952-0567c5846987" xlink:to="loc_srt_SegmentGeographicalDomain_098372e5-eb22-4b8c-a952-0567c5846987_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_ad8aba66-a2e9-48be-931c-eff4cea4d640" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_098372e5-eb22-4b8c-a952-0567c5846987" xlink:to="loc_srt_SegmentGeographicalDomain_ad8aba66-a2e9-48be-931c-eff4cea4d640" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_LocationOneMember_0c65f096-2a0a-4434-9b21-20cf3f736613" xlink:href="dco-20220402.xsd#dco_LocationOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_ad8aba66-a2e9-48be-931c-eff4cea4d640" xlink:to="loc_dco_LocationOneMember_0c65f096-2a0a-4434-9b21-20cf3f736613" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_LocationTwoMember_bde02999-ed78-4666-91e5-cde6df2d649c" xlink:href="dco-20220402.xsd#dco_LocationTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_ad8aba66-a2e9-48be-931c-eff4cea4d640" xlink:to="loc_dco_LocationTwoMember_bde02999-ed78-4666-91e5-cde6df2d649c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_GuaymasMexicoMember_5e219cae-e529-42de-a675-6523426cdccf" xlink:href="dco-20220402.xsd#dco_GuaymasMexicoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_ad8aba66-a2e9-48be-931c-eff4cea4d640" xlink:to="loc_dco_GuaymasMexicoMember_5e219cae-e529-42de-a675-6523426cdccf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_8057d8e1-f2c3-4449-94ba-446a25cd2df7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_a1353ab3-1b2b-4913-aef1-bfaec6242376" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_8057d8e1-f2c3-4449-94ba-446a25cd2df7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_8057d8e1-f2c3-4449-94ba-446a25cd2df7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_8057d8e1-f2c3-4449-94ba-446a25cd2df7" xlink:to="loc_us-gaap_SegmentDomain_8057d8e1-f2c3-4449-94ba-446a25cd2df7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_8ef58b5d-d919-4229-a99d-52da3f23c868" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_8057d8e1-f2c3-4449-94ba-446a25cd2df7" xlink:to="loc_us-gaap_SegmentDomain_8ef58b5d-d919-4229-a99d-52da3f23c868" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_StructuralSystemsMember_f85e73cd-1b2d-45fa-b5e0-0cdfd23f1603" xlink:href="dco-20220402.xsd#dco_StructuralSystemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_8ef58b5d-d919-4229-a99d-52da3f23c868" xlink:to="loc_dco_StructuralSystemsMember_f85e73cd-1b2d-45fa-b5e0-0cdfd23f1603" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_265470a4-04f7-472d-a693-eb3a3f3605b8" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_a1353ab3-1b2b-4913-aef1-bfaec6242376" xlink:to="loc_srt_RangeAxis_265470a4-04f7-472d-a693-eb3a3f3605b8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_265470a4-04f7-472d-a693-eb3a3f3605b8_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_265470a4-04f7-472d-a693-eb3a3f3605b8" xlink:to="loc_srt_RangeMember_265470a4-04f7-472d-a693-eb3a3f3605b8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_99b11242-1b9e-4f96-b2bb-ecd90e2c08af" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_265470a4-04f7-472d-a693-eb3a3f3605b8" xlink:to="loc_srt_RangeMember_99b11242-1b9e-4f96-b2bb-ecd90e2c08af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_80a8367a-c81e-40bf-ac23-e22715cba67d" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_99b11242-1b9e-4f96-b2bb-ecd90e2c08af" xlink:to="loc_srt_MinimumMember_80a8367a-c81e-40bf-ac23-e22715cba67d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_df10f214-5af6-4db9-9fde-81623fdd9361" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_99b11242-1b9e-4f96-b2bb-ecd90e2c08af" xlink:to="loc_srt_MaximumMember_df10f214-5af6-4db9-9fde-81623fdd9361" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_767c8d25-00a9-40e3-9199-aa6a4a99f649" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LossContingenciesByNatureOfContingencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_a1353ab3-1b2b-4913-aef1-bfaec6242376" xlink:to="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_767c8d25-00a9-40e3-9199-aa6a4a99f649" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyNatureDomain_767c8d25-00a9-40e3-9199-aa6a4a99f649_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LossContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_767c8d25-00a9-40e3-9199-aa6a4a99f649" xlink:to="loc_us-gaap_LossContingencyNatureDomain_767c8d25-00a9-40e3-9199-aa6a4a99f649_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyNatureDomain_3e53704b-bc51-4db7-b57d-d73b6cdc5c92" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LossContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_767c8d25-00a9-40e3-9199-aa6a4a99f649" xlink:to="loc_us-gaap_LossContingencyNatureDomain_3e53704b-bc51-4db7-b57d-d73b6cdc5c92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DamageFromFireExplosionOrOtherHazardMember_eb9fdad7-0166-4fe4-a5a6-3af6750b87b9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DamageFromFireExplosionOrOtherHazardMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingencyNatureDomain_3e53704b-bc51-4db7-b57d-d73b6cdc5c92" xlink:to="loc_us-gaap_DamageFromFireExplosionOrOtherHazardMember_eb9fdad7-0166-4fe4-a5a6-3af6750b87b9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementAxis_6032f360-5e3f-4c08-af20-98233cbbd4ce" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RestatementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_a1353ab3-1b2b-4913-aef1-bfaec6242376" xlink:to="loc_srt_RestatementAxis_6032f360-5e3f-4c08-af20-98233cbbd4ce" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_6032f360-5e3f-4c08-af20-98233cbbd4ce_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RestatementAxis_6032f360-5e3f-4c08-af20-98233cbbd4ce" xlink:to="loc_srt_RestatementDomain_6032f360-5e3f-4c08-af20-98233cbbd4ce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RestatementDomain_53f7d6e4-52a3-4980-b867-5d6b606690fa" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RestatementDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RestatementAxis_6032f360-5e3f-4c08-af20-98233cbbd4ce" xlink:to="loc_srt_RestatementDomain_53f7d6e4-52a3-4980-b867-5d6b606690fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RevisionOfPriorPeriodReclassificationAdjustmentMember_cb4f5cd5-cbaf-4e06-aab5-c1e901fa0ff9" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RevisionOfPriorPeriodReclassificationAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RestatementDomain_53f7d6e4-52a3-4980-b867-5d6b606690fa" xlink:to="loc_srt_RevisionOfPriorPeriodReclassificationAdjustmentMember_cb4f5cd5-cbaf-4e06-aab5-c1e901fa0ff9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_305bc610-7af6-4779-917b-350cfb334624" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByAssetClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_a1353ab3-1b2b-4913-aef1-bfaec6242376" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_305bc610-7af6-4779-917b-350cfb334624" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_305bc610-7af6-4779-917b-350cfb334624_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_305bc610-7af6-4779-917b-350cfb334624" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_305bc610-7af6-4779-917b-350cfb334624_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_87df018b-f165-447e-aa0f-70a9a5c1fb8c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_305bc610-7af6-4779-917b-350cfb334624" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_87df018b-f165-447e-aa0f-70a9a5c1fb8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentMember_62ea3dce-6477-490c-ac39-e1a25d565b57" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_87df018b-f165-447e-aa0f-70a9a5c1fb8c" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentMember_62ea3dce-6477-490c-ac39-e1a25d565b57" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoriesMember_84b93921-adc5-44b0-bc60-4220985f91c3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InventoriesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_87df018b-f165-447e-aa0f-70a9a5c1fb8c" xlink:to="loc_us-gaap_InventoriesMember_84b93921-adc5-44b0-bc60-4220985f91c3" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/BusinessSegmentInformationFinancialInformationbyReportableSegmentDetails" xlink:type="simple" xlink:href="dco-20220402.xsd#BusinessSegmentInformationFinancialInformationbyReportableSegmentDetails"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/BusinessSegmentInformationFinancialInformationbyReportableSegmentDetails" xlink:type="extended" id="idc884aeaba95420390c6ed9b7d6e37c9_BusinessSegmentInformationFinancialInformationbyReportableSegmentDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_23cb979e-1668-436e-bc67-b425a725731d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_49b0748f-5487-415d-806c-f0f7b15dfbe7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_23cb979e-1668-436e-bc67-b425a725731d" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_49b0748f-5487-415d-806c-f0f7b15dfbe7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_7ea97026-0c34-4a2c-aed8-758ab10cc987" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_23cb979e-1668-436e-bc67-b425a725731d" xlink:to="loc_us-gaap_OperatingIncomeLoss_7ea97026-0c34-4a2c-aed8-758ab10cc987" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_1e778211-c236-46b3-9414-b0aa6b7c494c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_23cb979e-1668-436e-bc67-b425a725731d" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_1e778211-c236-46b3-9414-b0aa6b7c494c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_968fdb84-34f1-4c40-9f9f-60f09e619e7e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_23cb979e-1668-436e-bc67-b425a725731d" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_968fdb84-34f1-4c40-9f9f-60f09e619e7e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_CapitalExpenditures_e667e16f-533a-4903-aa1f-defa5a3534d8" xlink:href="dco-20220402.xsd#dco_CapitalExpenditures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_23cb979e-1668-436e-bc67-b425a725731d" xlink:to="loc_dco_CapitalExpenditures_e667e16f-533a-4903-aa1f-defa5a3534d8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_c4ab4b84-8c48-4504-8765-ffd41443925e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_23cb979e-1668-436e-bc67-b425a725731d" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_c4ab4b84-8c48-4504-8765-ffd41443925e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_2c778003-162c-4d9e-9926-996eb19902fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_c4ab4b84-8c48-4504-8765-ffd41443925e" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_2c778003-162c-4d9e-9926-996eb19902fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_2c778003-162c-4d9e-9926-996eb19902fd_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_2c778003-162c-4d9e-9926-996eb19902fd" xlink:to="loc_us-gaap_SegmentDomain_2c778003-162c-4d9e-9926-996eb19902fd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_6f20140b-a5ad-43ff-bf69-4e13d28972d3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_2c778003-162c-4d9e-9926-996eb19902fd" xlink:to="loc_us-gaap_SegmentDomain_6f20140b-a5ad-43ff-bf69-4e13d28972d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_ElectronicSystemsMember_0dfa9001-0c8e-491c-8b36-e811016494e8" xlink:href="dco-20220402.xsd#dco_ElectronicSystemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_6f20140b-a5ad-43ff-bf69-4e13d28972d3" xlink:to="loc_dco_ElectronicSystemsMember_0dfa9001-0c8e-491c-8b36-e811016494e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_StructuralSystemsMember_aa31edab-6f2c-4290-8778-bc3378a514a0" xlink:href="dco-20220402.xsd#dco_StructuralSystemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_6f20140b-a5ad-43ff-bf69-4e13d28972d3" xlink:to="loc_dco_StructuralSystemsMember_aa31edab-6f2c-4290-8778-bc3378a514a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_343bc68c-5f57-4d54-a454-c5cd4b822485" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_c4ab4b84-8c48-4504-8765-ffd41443925e" xlink:to="loc_srt_ConsolidationItemsAxis_343bc68c-5f57-4d54-a454-c5cd4b822485" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_343bc68c-5f57-4d54-a454-c5cd4b822485_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_343bc68c-5f57-4d54-a454-c5cd4b822485" xlink:to="loc_srt_ConsolidationItemsDomain_343bc68c-5f57-4d54-a454-c5cd4b822485_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_59237af0-eeb9-44d3-84d3-0cd4100acd42" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_343bc68c-5f57-4d54-a454-c5cd4b822485" xlink:to="loc_srt_ConsolidationItemsDomain_59237af0-eeb9-44d3-84d3-0cd4100acd42" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_aa4928e4-5322-45e6-8771-6d787e92de16" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_59237af0-eeb9-44d3-84d3-0cd4100acd42" xlink:to="loc_us-gaap_OperatingSegmentsMember_aa4928e4-5322-45e6-8771-6d787e92de16" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MaterialReconcilingItemsMember_7d8f8180-3989-4872-a65d-9b8a6a6c6f0a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MaterialReconcilingItemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_59237af0-eeb9-44d3-84d3-0cd4100acd42" xlink:to="loc_us-gaap_MaterialReconcilingItemsMember_7d8f8180-3989-4872-a65d-9b8a6a6c6f0a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNonSegmentMember_0e794f20-ba80-4971-9ae3-6fca6568303b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CorporateNonSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_59237af0-eeb9-44d3-84d3-0cd4100acd42" xlink:to="loc_us-gaap_CorporateNonSegmentMember_0e794f20-ba80-4971-9ae3-6fca6568303b" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.ducommun.com/role/BusinessSegmentInformationSegmentAssetsDetails" xlink:type="simple" xlink:href="dco-20220402.xsd#BusinessSegmentInformationSegmentAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.ducommun.com/role/BusinessSegmentInformationSegmentAssetsDetails" xlink:type="extended" id="ib378878eb1c346428f3645b6b74e7b9d_BusinessSegmentInformationSegmentAssetsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_544fabf8-6541-455d-b08c-ecae753c3b57" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_02bb2300-e79c-4417-b091-3f864924a24c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_544fabf8-6541-455d-b08c-ecae753c3b57" xlink:to="loc_us-gaap_Assets_02bb2300-e79c-4417-b091-3f864924a24c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_GoodwillAndOtherIntangibleAssets_b1f4bfc5-7e3e-41a7-b898-e192269b8b5c" xlink:href="dco-20220402.xsd#dco_GoodwillAndOtherIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_544fabf8-6541-455d-b08c-ecae753c3b57" xlink:to="loc_dco_GoodwillAndOtherIntangibleAssets_b1f4bfc5-7e3e-41a7-b898-e192269b8b5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_8663eb52-ab00-4c03-b409-83fa9a9d1af9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_544fabf8-6541-455d-b08c-ecae753c3b57" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_8663eb52-ab00-4c03-b409-83fa9a9d1af9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_aeb4f5e7-52e3-4745-b6fc-da7319ffd25f" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_8663eb52-ab00-4c03-b409-83fa9a9d1af9" xlink:to="loc_srt_ConsolidationItemsAxis_aeb4f5e7-52e3-4745-b6fc-da7319ffd25f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_aeb4f5e7-52e3-4745-b6fc-da7319ffd25f_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_aeb4f5e7-52e3-4745-b6fc-da7319ffd25f" xlink:to="loc_srt_ConsolidationItemsDomain_aeb4f5e7-52e3-4745-b6fc-da7319ffd25f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_90d72f0f-1b67-41e4-bdf3-d1d3c72bc744" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_aeb4f5e7-52e3-4745-b6fc-da7319ffd25f" xlink:to="loc_srt_ConsolidationItemsDomain_90d72f0f-1b67-41e4-bdf3-d1d3c72bc744" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_8e20c48d-440e-4918-8206-14cf8550ea89" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_90d72f0f-1b67-41e4-bdf3-d1d3c72bc744" xlink:to="loc_us-gaap_OperatingSegmentsMember_8e20c48d-440e-4918-8206-14cf8550ea89" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNonSegmentMember_da7bccb2-5bcb-429d-a153-05b0f71cd166" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CorporateNonSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_90d72f0f-1b67-41e4-bdf3-d1d3c72bc744" xlink:to="loc_us-gaap_CorporateNonSegmentMember_da7bccb2-5bcb-429d-a153-05b0f71cd166" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_cca464c1-486f-477a-bf71-f3f075013b99" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_8663eb52-ab00-4c03-b409-83fa9a9d1af9" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_cca464c1-486f-477a-bf71-f3f075013b99" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_cca464c1-486f-477a-bf71-f3f075013b99_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_cca464c1-486f-477a-bf71-f3f075013b99" xlink:to="loc_us-gaap_SegmentDomain_cca464c1-486f-477a-bf71-f3f075013b99_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_cc578dc6-a861-4299-ad57-1ffd88339cdc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_cca464c1-486f-477a-bf71-f3f075013b99" xlink:to="loc_us-gaap_SegmentDomain_cc578dc6-a861-4299-ad57-1ffd88339cdc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_ElectronicSystemsMember_4cf4c06d-0b38-45c3-9fcf-de8802407176" xlink:href="dco-20220402.xsd#dco_ElectronicSystemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_cc578dc6-a861-4299-ad57-1ffd88339cdc" xlink:to="loc_dco_ElectronicSystemsMember_4cf4c06d-0b38-45c3-9fcf-de8802407176" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dco_StructuralSystemsMember_3d7c17ad-92b4-408b-bfdf-108d33fb4a89" xlink:href="dco-20220402.xsd#dco_StructuralSystemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_cc578dc6-a861-4299-ad57-1ffd88339cdc" xlink:to="loc_dco_StructuralSystemsMember_3d7c17ad-92b4-408b-bfdf-108d33fb4a89" xlink:type="arc" order="1"/>
  </link:definitionLink>
</link:linkbase>
