Plain English Meaning
The cockpit gauge that tells the pilot how high the airplane is flying. It works by sensing how much air pressure is pressing on it — the higher you go, the lower the pressure — and turning that into a height in feet.
Full Definition
A flight instrument that displays the airplane's altitude by measuring the surrounding atmospheric pressure and converting it into a height reading, typically referenced to mean sea level when set to the local altimeter setting.
Context Anchor
Seen on the instrument panel during preflight, taxi, takeoff, climb, , , and . During a , the pilot checks that the altimeter appears and shows a reasonable for the when set correctly.