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.
Plain English
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.
Context Anchor
Pilots use the aircraft altimeter during preflight checks, takeoff, climb, cruise, descent, approach, and whenever they set or verify an assigned altitude.
Derivation
From Latin altus (high) and the Greek-derived metron (measure) -- literally 'height measurer.' The name reflects what it does: it measures height, even though what it actually senses is pressure.
Why Pilots Care
Provides continuous altitude information required for airspace compliance, safe separation from terrain, and precise vertical navigation.
Analogy
Works like a barometer that has been calibrated to convert pressure readings into feet of height instead of just showing pressure.
Grounding Statement
As an aircraft climbs, outside air pressure normally decreases, and the aircraft altimeter turns that pressure change into a higher altitude reading.
Intuition Check
Do not assume an aircraft altimeter usually measures straight-down height above the ground. In normal use, it measures air pressure and converts it into an altitude reading based on the selected pressure setting.
Example Sentence 1
Before taxi, the pilot set the altimeter to the current local pressure setting and confirmed the reading matched the airport's published elevation.
Example Sentence 2
On final approach the aircraft altimeter read 800 feet when the runway environment came into view.