Plain English Meaning
An early model of the radar system that air traffic controllers at a busy airport use to see where airplanes are around the field. It shows direction and distance from the radar, but not how high the aircraft is flying.
Full Definition
An older generation of Airport Surveillance Radar used by terminal radar facilities to detect and display the position of aircraft within roughly 60 nautical miles of an airport. It provides azimuth and range information but does not provide altitude data on its own.