Over-collateralization means the assessed value of pledged assets exceeds the debt or other obligation they secure. In DeFi, it is commonly used for borrowing and some collateral-backed token issuance. The surplus provides a buffer against price changes and the cost of recovering funds if a position becomes unsafe.
Why borrowers provide extra assets
A borrower may want spending liquidity while retaining exposure to a cryptoasset's price. Depositing that asset as collateral can enable borrowing another asset without selling the original holding, although the collateral becomes subject to the protocol's controls and liquidation rules.
Protocols set eligible collateral and borrowing limits rather than treating every token at its full market value. More volatile or less liquid collateral may support less borrowing. Requirements can also differ between opening a position and keeping it above the liquidation boundary.
A buffer that can shrink
Falling collateral prices, rising debt-asset prices, or accrued interest can consume the surplus. A position may become liquidatable while its collateral still exceeds its debt, because the protocol requires an additional margin.
Extra collateral reduces some credit exposure but cannot guarantee repayment. Oracle errors, market gaps, limited liquidation liquidity, and contract failures can still cause losses. Maintaining the buffer may require adding collateral or reducing debt.