<?xml version="1.0" encoding="utf-8"?>
<!--XBRL document created with Toppan Merrill Bridge  11.3.0.1 -->
<!--Based on XBRL 2.1-->
<!--Created on: 6/1/2026 12:59:49 PM-->
<!--Modified on: 6/1/2026 12:59:49 PM-->
<link:linkbase xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt">
  <link:roleRef roleURI="http://xbrl.sec.gov/cef/role/SecurityOnly" xlink:href="https://xbrl.sec.gov/cef/2024/cef-2024.xsd#SecurityOnly" xlink:type="simple" />
  <link:roleRef roleURI="http://xbrl.sec.gov/cef/role/RiskOnly" xlink:href="https://xbrl.sec.gov/cef/2024/cef-2024.xsd#RiskOnly" xlink:type="simple" />
  <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/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: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://www.esma.europa.eu/xbrl/esef/arcrole/wider-narrower" xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/arcrole/esma-arcrole-2018-11-21.xsd#wider-narrower" />
  <link:definitionLink xlink:role="http://xbrl.sec.gov/cef/role/RiskOnly" xlink:type="extended" xlink:title="Risk">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/cef/2024/cef-2024.xsd#cef_AllRisksMember" xlink:label="cef_AllRisksMember" xlink:title="cef_AllRisksMember" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_RisksRelatedToNotesMember" xlink:label="fsk_RisksRelatedToNotesMember" xlink:title="fsk_RisksRelatedToNotesMember" />
    <link:definitionArc xlink:type="arc" xlink:from="cef_AllRisksMember" xlink:to="fsk_RisksRelatedToNotesMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:cef_AllRisksMember To fsk_RisksRelatedToNotesMember" order="1" priority="0" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/cef/role/SecurityOnly" xlink:type="extended" xlink:title="Security">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" xlink:title="us-gaap_ClassOfStockDomain" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" xlink:title="us-gaap_CommonStockMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonStockMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_ClassOfStockDomain To us-gaap_CommonStockMember" order="1" priority="0" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_PreferredSharesMember" xlink:label="fsk_PreferredSharesMember" xlink:title="fsk_PreferredSharesMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="fsk_PreferredSharesMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_ClassOfStockDomain To fsk_PreferredSharesMember" order="2" priority="0" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" xlink:title="us-gaap_DebtInstrumentNameDomain" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2024/elts/us-gaap-2024.xsd#us-gaap_WarrantMember" xlink:label="us-gaap_WarrantMember" xlink:title="us-gaap_WarrantMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="us-gaap_WarrantMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_DebtInstrumentNameDomain To us-gaap_WarrantMember" order="1" priority="0" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_SubscriptionRightsMember" xlink:label="fsk_SubscriptionRightsMember" xlink:title="fsk_SubscriptionRightsMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="fsk_SubscriptionRightsMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_DebtInstrumentNameDomain To fsk_SubscriptionRightsMember" order="2" priority="0" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_UnitsMember" xlink:label="fsk_UnitsMember" xlink:title="fsk_UnitsMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="fsk_UnitsMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_DebtInstrumentNameDomain To fsk_UnitsMember" order="3" priority="0" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_DebtNote4.250InterestRateMember" xlink:label="fsk_DebtNote4.250InterestRateMember" xlink:title="fsk_DebtNote4.250InterestRateMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="fsk_DebtNote4.250InterestRateMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_DebtInstrumentNameDomain To fsk_DebtNote4.250InterestRateMember" order="4" priority="0" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_DebtNote1.650InterestRateMember" xlink:label="fsk_DebtNote1.650InterestRateMember" xlink:title="fsk_DebtNote1.650InterestRateMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="fsk_DebtNote1.650InterestRateMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_DebtInstrumentNameDomain To fsk_DebtNote1.650InterestRateMember" order="5" priority="0" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_DebtNote4.125InterestRateMember" xlink:label="fsk_DebtNote4.125InterestRateMember" xlink:title="fsk_DebtNote4.125InterestRateMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="fsk_DebtNote4.125InterestRateMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_DebtInstrumentNameDomain To fsk_DebtNote4.125InterestRateMember" order="6" priority="0" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_DebtNote8.625InterestRateMember" xlink:label="fsk_DebtNote8.625InterestRateMember" xlink:title="fsk_DebtNote8.625InterestRateMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="fsk_DebtNote8.625InterestRateMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_DebtInstrumentNameDomain To fsk_DebtNote8.625InterestRateMember" order="7" priority="0" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_DebtNote3.400InterestRateMember" xlink:label="fsk_DebtNote3.400InterestRateMember" xlink:title="fsk_DebtNote3.400InterestRateMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="fsk_DebtNote3.400InterestRateMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_DebtInstrumentNameDomain To fsk_DebtNote3.400InterestRateMember" order="8" priority="0" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_DebtNote2.625InterestRateMember" xlink:label="fsk_DebtNote2.625InterestRateMember" xlink:title="fsk_DebtNote2.625InterestRateMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="fsk_DebtNote2.625InterestRateMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_DebtInstrumentNameDomain To fsk_DebtNote2.625InterestRateMember" order="9" priority="0" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_DebtNote3.250InterestRateMember" xlink:label="fsk_DebtNote3.250InterestRateMember" xlink:title="fsk_DebtNote3.250InterestRateMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="fsk_DebtNote3.250InterestRateMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_DebtInstrumentNameDomain To fsk_DebtNote3.250InterestRateMember" order="10" priority="0" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_DebtNote3.125InterestRateMember" xlink:label="fsk_DebtNote3.125InterestRateMember" xlink:title="fsk_DebtNote3.125InterestRateMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="fsk_DebtNote3.125InterestRateMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_DebtInstrumentNameDomain To fsk_DebtNote3.125InterestRateMember" order="11" priority="0" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_DebtNote7.875InterestRateMember" xlink:label="fsk_DebtNote7.875InterestRateMember" xlink:title="fsk_DebtNote7.875InterestRateMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="fsk_DebtNote7.875InterestRateMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_DebtInstrumentNameDomain To fsk_DebtNote7.875InterestRateMember" order="12" priority="0" />
    <link:loc xlink:type="locator" xlink:href="tmb-20260527.xsd#fsk_DebtNote6.875InterestRateMember" xlink:label="fsk_DebtNote6.875InterestRateMember" xlink:title="fsk_DebtNote6.875InterestRateMember" />
    <link:definitionArc xlink:type="arc" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="fsk_DebtNote6.875InterestRateMember" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:title="definition:us-gaap_DebtInstrumentNameDomain To fsk_DebtNote6.875InterestRateMember" order="13" priority="0" />
  </link:definitionLink>
</link:linkbase>
