Plain English Meaning
The piece of ground radar equipment that sends a signal asking each aircraft, "Who are you and how high are you?" The aircraft's transponder hears the question and sends back the answer.
Full Definition
The ground-based radar component that transmits coded pulse signals to an aircraft's transponder, prompting it to reply with identification and altitude information. The interrogating unit is the ground side of the secondary surveillance radar (SSR) system; the transponder is the airborne side that answers it.
Context Anchor
Seen in ground-based radar navigation and distance-measuring equipment discussions, especially when describing how an aircraft gets distance information from a navigation station.
Derivation
From the Latin interrogare, meaning "to question" or "ask." The name exactly what the unit does: it asks the aircraft a question and waits for a reply.