Plain English Meaning

An instrument in the cockpit that tells the pilot how high the aircraft is by reading the air pressure outside. Higher up, the air is thinner, so lower pressure means a higher reading.

Full Definition

A flight instrument that measures the aircraft's height above a selected pressure reference by sensing the static air pressure outside the aircraft and converting it into an altitude reading in feet. The pilot sets a reference pressure (in inches of mercury or hectopascals) in the instrument's setting window, which adjusts what the indicated altitude represents -- typically height above mean sea level when set to the local altimeter setting.