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.
Plain English
A small device in the aircraft that answers radar signals so controllers can see who you are and how high you are flying.
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.