A published attestation that a custodian holds assets corresponding to customer balances, usually via a cryptographic commitment to the liability set plus evidence of controlled addresses.
Why it matters
Proof of reserves demonstrates assets. On its own it does not demonstrate solvency, because it says nothing about liabilities outside the attested set — borrowings, obligations to other parties, or off-chain debt. A reserves attestation without a corresponding liabilities attestation is a partial picture.
Not advice. Definitions are for understanding, not instruction. Nothing here is financial, legal, tax or security advice.