Plain English Meaning

ALT is just shorthand for 'altitude' — how high the aircraft is. On a transponder, selecting ALT tells the box to send the aircraft's height to ATC along with its identifying code.

Full Definition

ALT is the standard abbreviation for altitude — the vertical distance of an aircraft, measured from a stated reference such as mean sea level or the ground. In the context of Mode C, ALT refers specifically to the pressure altitude data that the transponder automatically reports to air traffic control radar.

Context Anchor

Seen on transponder controls and in discussions of Mode C altitude reporting.