Generally, it is the date a transaction is completed, i.e., the date that performance (the actual exchange: money vs asset or liability) by the parties to the transaction has been duly fulfilled.
In accounting, it is the date that a financial asset is delivered to or by an entity, or on which a financial liability is transferred to or by an entity. It is the closing date on which delivery or transfer is recorded under the so-called settlement-date accounting. This type of accounting is a conservative methodolgy, since it results in a delay of a few days before a transaction is officially recorded.
