A category of securities that are eligible to be posted as collateral to specific banking transactions such as repurchase agreement (repos). Such securities include treasury securities, agency debts, and marketable mortgage-backed securities (MBSs).

Eligible securities usually have better (i.e., lower) haircuts for collateralization compared to non-eligible securities, particularly those with no market price, no good liquidity, and no obvious method of estimating the market price.