Definition
The transponder's automatic radio response to an interrogation from a ground-based secondary surveillance radar, transmitted as a specific pulse pattern that encodes the assigned four-digit squawk code and, when Mode C or Mode S is active, the aircraft's pressure altitude.
Plain English
When ground radar 'pings' your aircraft, your transponder automatically sends back a short signal carrying your assigned code and altitude. That return signal is the coded reply.
Context Anchor
Seen in radar and transponder discussions, especially when explaining how air traffic control identifies an aircraft on a radar display.
Derivation
Coded' because the reply isn't a voice message but a pattern of pulses representing numbers. 'Reply' because the transponder only speaks when spoken to -- it answers a radar interrogation rather than transmitting on its own.
Why Pilots Care
Enables ATC to positively identify your aircraft, maintain separation from other traffic, and provide radar services.
Intuition Check
Do not read “reply” as something the pilot says on the radio. Here, the reply is an automatic electronic response sent by the aircraft’s transponder.
Example Sentence 1
After the controller assigned squawk 4271, the transponder's coded reply allowed the radar to tag the aircraft with its callsign and altitude.
Example Sentence 2
When interrogated by secondary radar, the coded reply included Mode C altitude data for vertical separation.