Plain English Meaning
A small box in the cockpit that talks to ATC's radar. When the radar pings the aircraft, the transponder pings back with an identifying code and altitude, so controllers can see exactly which dot on their screen is which aircraft.
Full Definition
A radio device installed in the aircraft that automatically replies to interrogations from ground-based secondary surveillance radar. When triggered, it transmits a coded signal back to air traffic control, identifying the aircraft and (in Mode C or Mode S) reporting its pressure altitude.
Context Anchor
Seen on avionics displays, transponder controls, and cockpit checklists when setting or checking the aircraft’s assigned .