Definition
An onboard avionics unit that receives interrogation signals from ground-based secondary surveillance radar and automatically transmits a coded reply, allowing air traffic control to identify the aircraft on radar and, when Mode C or Mode S is active, to display its pressure altitude.
Plain English
A radio box in the aircraft that answers radar pulses from air traffic control. Each reply tags your aircraft on the controller's screen with a unique four-digit code and, in most modes, your altitude.
Context Anchor
Seen on avionics displays, panel controls, and checklists, including instrument-panel figures where the transponder control or display is labeled XPDR.
Derivation
From "transmitter" + "responder." The unit responds to a transmission rather than broadcasting on its own — it stays quiet until interrogated by radar, then answers.
Why Pilots Care
It keeps the aircraft visible to controllers, supports traffic separation, and satisfies regulatory requirements in controlled airspace.
Analogy
A transponder is a little like caller ID for an aircraft: it helps the controller know which aircraft is showing on the screen, rather than seeing only an unidentified return.
Intuition Check
Do not read XPDR as a separate system from the transponder. XPDR is simply the shortened panel label for the aircraft’s transponder.
Example Sentence 1
Before takeoff, the pilot set the XPDR to the assigned squawk code and switched it to ALT so ATC would receive the altitude readout.
Example Sentence 2
ATC asked the pilot to confirm the transponder was operating in altitude reporting mode.