Transponder

n. device that automatically transmits when it receives a signal

In telecommunication, the term transponder (short-for Transmitter-responder and sometimes abbreviated to XPDR, XPNDR or TPDR) has the following meanings:An automatic device that receives, amplifies, and retransmits a signal on a different frequency (see also broadcast translator). An automatic device that transmits a predetermined message in response to a predefined received signal. A receiver-transmitter that will generate a reply signal upon proper electronic interrogation.
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Noun
1. electrical device designed to receive a specific signal and automatically transmit a specific reply
(hypernym) electrical device

1 . An automatic device that receives, amplifies, and retransmits a signal on a different frequency. (188 ) 2. An automatic device that transmits a predetermined message in response to a predefined received signal. (188 ) Note: An example of transponders is in identification-friend-or-foe systems and air-traffic-control secondary radar (beacon radar) systems. 3. A receiver-transmitter that will generate a reply signal upon proper interrogation. [JP1]

The airborne radar beacon receiver/transmitter portion of the Air Traffic Control Beacon System that automatically receives radio signals from interrogators on the ground and selectively replaces with specific reply pulse-on-pulse group only those interrogations being received on the mode to which it is set to respond. Each aircraft transponder is capable of replying to 4,096 codes as selected by the pilot. Provides the air traffic controller positive location and, in some cases, altitude information. (FAA6)

A piece of equipment which, when set upon a specific radio frequency, emits a directional signal of its own and enables the receiver to track or monitor the truck's location. (BOC3)