Final answer:
A rollback occurs when a transaction fails after updating an item in a database, reverting the database to its state before the start of the transaction to maintain consistency.
Step-by-step explanation:
When a transaction updates a database item and then fails, a rollback is the process that ensures database integrity is maintained. The rollback operation undoes all the operations performed by the transaction that failed, thereby restoring the database to its previous state before the transaction began. This is crucial to preserve the atomicity property of transactions, which states that a transaction must be completed in its entirety or not at all. A commit, contrastingly, is the operation that would make all changes of a successful transaction permanent. A deadlock is a different concept where two or more transactions are waiting indefinitely for one another to release locks. This requires a deadlock resolution mechanism to resolve. Lastly, a partial update is an undesirable situation that rollback prevents, where only some changes of a transaction are applied, leading to an inconsistent state.