Explain what is meant by a transaction in database management systems.

A transaction in database management systems is a logically complete sequence of operations executed as a single unit. It has four main properties (ACID):

• Atomicity: all operations of a transaction are either fully completed or not executed at all.
• Consistency: after the transaction, the database remains in a correct state.
• Isolation: concurrent transactions do not affect each other as if they were executed sequentially.
• Durability: once committed, the transaction's changes are permanent, even in case of failures.

For example, in transferring money between accounts, a transaction guarantees that the debit from one account and credit to another happen together or not at all.