A transponder is a wireless communications, monitoring, or control device that picks up and responds to an incoming signal . The term is a contraction of the words trans mitter and res ponder .
Transponders can be either passive or active.
A passive transponder allows a computer or robot to identify an object. Magnetic labels, such as those on credit cards and store items, are common examples. A passive transponder must be used with an active sensor that decodes and transcribes the data the transponder contains. The transponder unit can be physically tiny, and its information can be sensed up to several feet away. Simple active transponders are employed in location, identification, and navigation systems.