Other forms: transponders
A transponder is a communication device that can either send or receive signals (or both). With a special transponder in your car, you can drive slowly through a tollbooth as your toll is collected electronically.
The word transponder is what's known as a portmanteau, or a combination of two words into one — in this case, it comes from transmit and respond. Airplanes carry transponders that can communicate with the radar used by air traffic controllers on the ground, and race cars contain transponders that help time their laps accurately.