<?xml version="1.0" encoding="utf-8"?>
<!--Generated by DFIN XBRL Instance Document - http://www.dfinsolutions.com/ - Version R3.0 - on 07-March-2023 [12:40:57] {AM}-->
<linkbase xmlns="http://www.xbrl.org/2003/linkbase" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xlink="http://www.w3.org/1999/xlink" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:xbrldt="http://xbrl.org/2005/xbrldt">
<roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/cef/2022/cef-2022.xsd#SecurityOnly" roleURI="http://xbrl.sec.gov/cef/role/SecurityOnly" />
<roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/cef/2022/cef-2022.xsd#RiskOnly" roleURI="http://xbrl.sec.gov/cef/role/RiskOnly" />
<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" />
<definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/cef/role/SecurityOnly">
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/cef/2022/cef-2022.xsd#cef_AllSecuritiesMember" xlink:label="loc_cef_AllSecuritiesMember_200072" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_CommonStockMember" xlink:label="loc_cik0001160864_CommonStockMember_200597" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllSecuritiesMember_200072" xlink:to="loc_cik0001160864_CommonStockMember_200597" use="optional" order="1" />
</definitionLink>
<definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/cef/role/RiskOnly">
<loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/cef/2022/cef-2022.xsd#cef_AllRisksMember" xlink:label="loc_cef_AllRisksMember_200074" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_InvestmentAndMarketDiscountRiskMember" xlink:label="loc_cik0001160864_InvestmentAndMarketDiscountRiskMember_200598" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_InvestmentAndMarketDiscountRiskMember_200598" use="optional" order="2" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_DebtSecuritiesRiskMember" xlink:label="loc_cik0001160864_DebtSecuritiesRiskMember_200599" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_DebtSecuritiesRiskMember_200599" use="optional" order="3" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_MortgageAndAssetbackedSecuritiesRisksMember" xlink:label="loc_cik0001160864_MortgageAndAssetbackedSecuritiesRisksMember_200600" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_MortgageAndAssetbackedSecuritiesRisksMember_200600" use="optional" order="4" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_U.s.GovernmentObligationsRiskMember" xlink:label="loc_cik0001160864_U.s.GovernmentObligationsRiskMember_200601" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_U.s.GovernmentObligationsRiskMember_200601" use="optional" order="5" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_RisksOfLoanAssignmentsAndParticipationsMember" xlink:label="loc_cik0001160864_RisksOfLoanAssignmentsAndParticipationsMember_200602" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_RisksOfLoanAssignmentsAndParticipationsMember_200602" use="optional" order="6" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_JunkBondsRiskMember" xlink:label="loc_cik0001160864_JunkBondsRiskMember_200603" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_JunkBondsRiskMember_200603" use="optional" order="7" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_ForeignSecuritiesRiskMember" xlink:label="loc_cik0001160864_ForeignSecuritiesRiskMember_200604" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_ForeignSecuritiesRiskMember_200604" use="optional" order="8" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_PreferredSecuritiesRiskMember" xlink:label="loc_cik0001160864_PreferredSecuritiesRiskMember_200605" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_PreferredSecuritiesRiskMember_200605" use="optional" order="9" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_ConvertibleSecuritiesRiskMember" xlink:label="loc_cik0001160864_ConvertibleSecuritiesRiskMember_200606" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_ConvertibleSecuritiesRiskMember_200606" use="optional" order="10" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_WarrantsRiskMember" xlink:label="loc_cik0001160864_WarrantsRiskMember_200607" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_WarrantsRiskMember_200607" use="optional" order="11" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_DerivativesRiskMember" xlink:label="loc_cik0001160864_DerivativesRiskMember_200608" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_DerivativesRiskMember_200608" use="optional" order="12" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_LeverageRiskMember" xlink:label="loc_cik0001160864_LeverageRiskMember_200609" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_LeverageRiskMember_200609" use="optional" order="13" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_ReverseRepurchaseAgreementsRiskMember" xlink:label="loc_cik0001160864_ReverseRepurchaseAgreementsRiskMember_200610" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_ReverseRepurchaseAgreementsRiskMember_200610" use="optional" order="14" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_DollarRollsRiskMember" xlink:label="loc_cik0001160864_DollarRollsRiskMember_200611" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_DollarRollsRiskMember_200611" use="optional" order="15" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_IlliquidInvestmentsRiskMember" xlink:label="loc_cik0001160864_IlliquidInvestmentsRiskMember_200612" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_IlliquidInvestmentsRiskMember_200612" use="optional" order="16" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_InvestmentCompaniesAndEtfsRiskMember" xlink:label="loc_cik0001160864_InvestmentCompaniesAndEtfsRiskMember_200613" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_InvestmentCompaniesAndEtfsRiskMember_200613" use="optional" order="17" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_MarketRiskMember" xlink:label="loc_cik0001160864_MarketRiskMember_200621" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_MarketRiskMember_200621" use="optional" order="18" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_InfectiousIllnessRiskMember" xlink:label="loc_cik0001160864_InfectiousIllnessRiskMember_200622" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_InfectiousIllnessRiskMember_200622" use="optional" order="19" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_ValuationRiskMember" xlink:label="loc_cik0001160864_ValuationRiskMember_200623" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_ValuationRiskMember_200623" use="optional" order="20" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_CounterpartyCreditRiskMember" xlink:label="loc_cik0001160864_CounterpartyCreditRiskMember_200624" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_CounterpartyCreditRiskMember_200624" use="optional" order="21" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_ConcentrationRiskMember" xlink:label="loc_cik0001160864_ConcentrationRiskMember_200625" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_ConcentrationRiskMember_200625" use="optional" order="22" />
<loc xlink:type="locator" xlink:href="cik0001160864-20230306.xsd#cik0001160864_LiborTransitionRiskMember" xlink:label="loc_cik0001160864_LiborTransitionRiskMember_200626" />
<definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cef_AllRisksMember_200074" xlink:to="loc_cik0001160864_LiborTransitionRiskMember_200626" use="optional" order="23" />
</definitionLink>
</linkbase>
