Plain English Meaning
Radar is the equipment on the ground that finds aircraft using radio waves. A transponder is the small unit in the aircraft that hears the radar and answers back with a code so the controller knows which aircraft it is and how high it is flying.
Full Definition
A paired system used in air traffic control where ground-based radar sends out radio pulses to detect aircraft, and an aircraft-mounted transponder receives those pulses and sends back a coded reply that identifies the aircraft and reports its altitude. Together they allow controllers to see, identify, and track aircraft on radar displays.
Context Anchor
You see this term when studying instrument flying equipment, air traffic control , displays, and .