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.
Derivation
From 'transmitter' + 'responder.' The device responds to an incoming signal by transmitting a coded reply — the name describes exactly what it does.
Why Pilots Care
Transponders are required in most controlled airspace and give ATC the information needed to identify and separate traffic.
Example Sentence 1
Before takeoff, the pilot set the transponder to the four-digit code assigned by ground control.
Example Sentence 2
ATC instructed the pilot to turn the transponder to altitude mode for the approach into Class C airspace.