In general terms, a property right created either by means of an agreement between parties or by operation of law over assets (or collateral) to secure the performance of an obligation or the payment of a debt.

For the purposes of the Personal Property Securities Act 1999 (PPSA 1999), a security interest is an interest in personal property that in substance secures the payment or performance of an obligation, or is otherwise a transaction that is deemed to be a security interest (section 17, PPSA 1999).