2 Answers

In aircraft terms it is a transmitter and receiver that sends a 'blip' to radar (FBO) with the aircrafts ID so that they can track your location.  They will sometimes ask the pilot to 'squawk'  so that they can identify as well..  means 'send a signal'.. All 'controlled' airspace now I believe requires transponder identification. at one time, this was not necessary. 

today in some cities they have bridge and road tolls, for regular commuters their cars are also equipped with a form of transponder that carries your cars identification, you do not need to stop to pay a toll, they send you a bill at the end of the month.. here it is called 'fast track'

It is also the little white thing that some of us have on the windshield in order to pay the tolls