An issuer reserve asset is any asset that a stablecoin or electronic money issuer holds to back the tokens it has in circulation, and the subject is covered in full under stablecoin reserves. The phrase refers to a single item in the pool, such as a bank deposit, a short-term government bill or a reverse repurchase agreement, while "stablecoin reserves" refers to the whole pool and the rules around it.
The term often appears in legislation and attestation reports, which list which kinds of asset count as eligible reserves and in what proportions. Common eligible assets are cash at regulated banks, short-dated government securities and money market fund shares that invest only in such instruments. Rules generally require reserves to equal at least the value of tokens outstanding, to be kept separate from the issuer's own funds, and to be verified through a regular reserve attestation.
The quality and liquidity of each reserve asset determine whether a stablecoin issuer can meet redemptions in a hurry; a reserve that cannot be sold quickly at full value increases the risk of a stablecoin depeg.