Definition
A receiver/transmitter carried in the aircraft that, on receiving a coded interrogation pulse from a ground-based secondary surveillance radar (SSR), automatically transmits a coded reply. The reply identifies the aircraft and, when Mode C or Mode S equipped, reports its pressure altitude and other data, allowing ATC to track and identify the aircraft on radar. This is the ICAO definition of the term.
Plain English
A box in the aircraft that listens for radar signals from the ground and answers back with a code that tells controllers which aircraft this is and how high it is flying.
Context Anchor
You encounter this term when setting a transponder code before flight, talking with air traffic control, operating in controlled airspace, or using traffic-alert equipment.
Derivation
From 'transmitter' + 'responder.' The word itself describes what the device does: it transmits a response when interrogated. Knowing this makes the function obvious — it doesn't broadcast on its own; it answers when asked.
Why Pilots Care
Allows air traffic controllers to identify and track the aircraft on radar for safe separation and collision avoidance.
Intuition Check
A transponder is not just a radio you talk on. It automatically replies to electronic requests with coded information about the aircraft.
Example Sentence 1
Before takeoff, the pilot set the transponder to the assigned code and selected ALT so that ATC would receive both identification and altitude.
Example Sentence 2
ATC asked the pilot to change the transponder code to confirm the aircraft's position.