A transponder is a device which replies to an interrogative transmission (Transmitter-Responder). It is used in civil aviation to allow ATC radar signals to trigger an informative broadcast from aircraft in their search area. They are used to track animals released into the wild. Some are used as navigation beacons.

The term is also used to refer to a paired transmitter and receiver on a satellite, usually because the satellite need only receive a simple confirmation signal that the listener is properly aligned before the associated relay transmitter begins to send data.