In one sentence
The contract between two lenders to the same borrower or project that fixes priority, payment rights, and who can act when things go wrong.
The longer version
Senior-mezzanine intercreditors govern cure rights, purchase options, and the mezz lender's ability to foreclose on equity. Senior-subordinate agreements inside participations and syndications allocate payments and control of remedies.
These agreements matter exactly when everything else is failing, which is why they get negotiated hard while everyone still likes each other.
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