Plain English Meaning
A ground radar that finds aircraft by bouncing radio waves off them and reading the echo. The aircraft does not need to do anything for the radar to see it.
Full Definition
A radar system that detects aircraft and other objects by transmitting radio energy and receiving the energy reflected back from the target. It determines the target's range and bearing from the timing and direction of the returned signal, without requiring any equipment in the aircraft itself.
Context Anchor
Pilots encounter this term in radar service, traffic advisories, and discussions of how air traffic controllers see aircraft on their displays.