A transponder (short-for Transmitter-responder and sometimes abbreviated to XPDR, XPNDR, TPDR or TP) is an electronic device that produces a response when it receives a radio-frequency interrogation.
In aviation, aircraft have transponders to assist in identifying them on radar and on other aircraft's collision avoidance systems.
In automotive applications, many modern automobiles have keys with transponders hidden inside the plastic head of the key. When the key is inserted into the ignition lock cylinder and turned, the car's computer sends a radio signal to the transponder. Unless the transponder replies with a valid code, the computer will not allow the engine to be started. Transponder keys have no battery; they are energized by the radio signal itself.