Dictionary.net

A set of related changes applied to a database. Theterm typically implies that either all of the changes should beapplied or, in the event of an error, none of them, i.e. thetransaction should be atomic. Atomicity is one of the ACIDproperties a transaction can have, another is isolation -preventing interference between processes trying to access thedatabase cocurrently. This is usually achieved by some form oflocking - where one process takes exclusive control of adatabase table or row for the duration of the transaction,preventing other processes from accessing the locked data.The canonical example of a transaction is transferring moneybetween two bank accounts by subtracting it from one and adding itto the other.Some relational database management systems require the user toexplicitly start a transaction and then either commit it (if allthe individual steps are successful) or roll it back (if there areany errors).