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.
Plain English
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.
Context Anchor
Seen in instrument flying, GPS discussions, preflight checks, approaches, and any time a pilot compares GPS altitude with the aircraft’s primary altitude instrument.
Derivation
From Greek baros (weight, pressure) + metron (measure), plus altimeter (Latin altus, high + meter). Literally a 'pressure measurer used to measure height.' The name itself tells you the instrument infers altitude indirectly — by measuring pressure, not height.
Why Pilots Care
Barometric altitude is the legal altitude reference for IFR flight, ATC clearances, and altitude separation. It must be set correctly with the current local altimeter setting (or 29.92 above the transition altitude) or the aircraft will fly at the wrong height relative to terrain and traffic. GPS altitude, by contrast, is geometric and is not used for altitude separation.
Intuition Check
Do not assume a barometric altimeter directly measures distance above the ground like a tape measure. It estimates altitude from air pressure, so the pressure setting matters.
Example Sentence 1
Before departure, the pilot set the local altimeter setting in the barometric altimeter so the indicated altitude matched the field elevation.
Example Sentence 2
While flying in clouds, the pilot relied on the barometric altimeter to maintain the assigned altitude.