Plain English Meaning
An altitude gauge that works by sensing air pressure. As the aircraft climbs, air pressure drops; the instrument reads that drop and shows it as a height in feet.
Full Definition
An aircraft instrument that determines altitude by measuring the surrounding atmospheric (static) pressure and converting it to a height reading above a selected pressure reference, typically displayed in feet. The pilot sets a reference pressure in the Kollsman window (e.g., 29.92 inHg or local altimeter setting), and the instrument indicates altitude relative to that reference based on the standard atmospheric pressure lapse rate.
Context Anchor
Seen in instrument flying, discussions, checks, , and any time a compares GPS altitude with the aircraft’s altitude instrument.