(1) A function that is registered by the application programmer to be called by the system or by the application when certain events occur in the system or application.
(2) A software routine that controls a program's reaction to specific external events, such as an interrupt handler.
(3) In the CICS/ESA Front End Programming Interface (FEPI), a transaction initiated to handle specified events.
(4) In Web services, a mechanism for processing service content and extending the function of a JAX-RPC runtime system.