Definition
A small indicator light on the transponder control panel that illuminates each time the transponder responds to an interrogation signal from a ground-based or airborne radar.
Plain English
A light on the transponder that flashes whenever the transponder is being seen by radar and is answering it.
Context Anchor
Seen on or near the transponder controls when checking whether the aircraft is being detected by radar equipment.
Derivation
From 'reply' (an answer) and 'light' (an indicator). The transponder literally replies to a radar interrogation, and the light shows that reply is happening.
Why Pilots Care
Confirms the transponder is receiving and responding to interrogations, verifying proper operation and preventing loss of radar contact or identity errors.
Intuition Check
Do not read “reply” as a spoken answer from air traffic control. Here it means the transponder has sent an automatic electronic response.
Example Sentence 1
After selecting ALT mode, the pilot watched the reply light flicker steadily, confirming the transponder was responding to ATC radar.
Example Sentence 2
During the preflight check the pilot confirmed the reply light illuminated when the transponder was placed in ALT mode.