Plain English Meaning
The dial in the cockpit that tells you how high you are flying. It works by measuring the air pressure outside the aircraft and turning that into an altitude number you can read.
Full Definition
The cockpit instrument that displays the aircraft's altitude above a selected pressure reference, typically mean sea level when set to the local altimeter setting. It senses changes in atmospheric pressure through a static port and converts those pressure changes into an altitude reading shown on its face.
Context Anchor
Seen on the instrument panel and in instrument-flying references when checking whether the aircraft is holding the assigned altitude.