Plain English Meaning
An aircraft carrying a small radio box that automatically answers radar with an ID code and altitude, so controllers and other aircraft can see exactly who and where it is.
Full Definition
An aircraft fitted with a working transponder — an onboard radio device that receives interrogation signals from ground or airborne radar and replies with a coded signal containing identity (squawk code) and, if Mode C or Mode S, pressure altitude. This reply allows air traffic control and other suitably equipped aircraft to detect, identify, and track the aircraft on radar displays.
Context Anchor
Seen in traffic avoidance discussions, especially when explaining which aircraft can be detected by - and air traffic control radar.