Plain English Meaning

A transponder setting that sends out a four-digit code so air traffic control can identify which target on their radar screen is your aircraft. It does not send altitude — that is a separate mode.

Full Definition

A transponder operating mode that transmits a four-digit identification code (0000–7777 in octal) when interrogated by ATC radar. Mode A returns identity only and does not include altitude information. The civilian designation is Mode A; the military designation for the same function is Mode 3, and the two are interoperable, which is why they are written together as Mode A/3.

Context Anchor

Seen on transponder controls and in discussions of radar identification, assigned codes, and transponder operation.

Derivation

"Mode" comes from the Latin modus, meaning "manner" or "method" — here, a method of replying to a radar . The letter A is the civilian label and the number 3 is the military label for the same identity-reporting function, dating to early shared civil/military .