In a DBMS, a trigger is a SQL procedure that initiates an action when an DML operation (INSERT, DELETE or UPDATE) occurs. So we can say that triggers are event-driven specialized procedures, they are stored in and managed by the DBMS.

A trigger cannot be called or executed; the DBMS automatically fires the trigger as a result of a data modification to the associated table. Triggers are used to maintain the referential integrity of data by changing the data in a systematic fashion.