Plain English Meaning
The ground radar equipment that asks each aircraft 'who are you and what altitude are you at?' Your transponder answers automatically, and the controller sees the result on the radar screen.
Full Definition
The ground-based surveillance radar transmitter that sends a coded signal to an aircraft's transponder, prompting the transponder to reply with identification and altitude information. Together with the airborne transponder, the interrogator forms the secondary surveillance radar (SSR) system that controllers use to track aircraft.
Context Anchor
Seen in radar, transponder, Mode A, Mode C, and Mode S discussions, especially when describing how air traffic control receives aircraft information.