The execution of business operations consisting of one or several requests or updates to a system as discrete units of work, each of them a transaction.
A transaction has a start and it is finished
by either a commit-command to activate all its changes made to the system
or by a rollback-command to roll back the system to the previous state
That means: A transaction either processes all of its changes or none of them. After the execution the system is always left in a valid state.
Transactions have 4 basic properties - the so called ACID properties
A = Atomicity (a transaction is completely finished or completely rolled back)
C = Consistency (system gets a valid new state or stays in the previous state)
I = Isolation (transaction is separated from other ones during processing)
D = Durability (system is left in a valid state even after a system failure)