Highest-priority security interest in a defined collateral pool. In a liquidation or restructuring, first-lien claims are repaid in full before any second-lien, mezzanine, or equity claim receives a dollar; the priority is enforced through UCC perfection on personal property and recorded mortgages on real estate, with relative priority among lien classes governed by an Inter-Creditor Agreement (ICA). Historical first-lien middle-market recovery is sim 70%, vs. sim 40% for second-lien and sim 20% for mezzanine.