Plain English Meaning
A small device in the aircraft that answers radar signals so controllers can see who you are and how high you are flying.
Full Definition
Airborne electronic devices that receive interrogation signals from ground-based radar and automatically reply with a coded signal, allowing air traffic control to identify the aircraft and, in most modes, display its altitude on the controller's radar screen.
Context Anchor
Pilots encounter transponders when setting a code assigned by air traffic control, using altitude reporting, or following checklist items for takeoff, landing, and flight in controlled airspace.