Plain English Meaning
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.
Full 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.
Context Anchor
Seen on avionics displays, panel controls, and , including -panel figures where the or display is labeled .