Definition
A flight instrument that displays the aircraft'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.
Plain English
The instrument in the cockpit that tells you how high you are flying.
Context Anchor
Seen in aircraft instrument descriptions, cockpit checks, instrument scans, and FAA abbreviation lists such as NOTAM contractions.
Derivation
From Latin altus meaning 'high' and the Greek metron meaning 'measure'. Literally a 'height-measurer', which is exactly what the instrument does.
Why Pilots Care
It lets pilots maintain assigned altitudes, avoid terrain, and comply with ATC clearances and airspace rules.
Intuition Check
An altimeter does not directly measure the distance from the aircraft to the ground. It measures air pressure and converts that pressure into an altitude indication based on the selected setting.
Example Sentence 1
Before takeoff, the pilot set the altimeter to the current pressure setting given by the tower.
Example Sentence 2
On final approach the altimeter read 800 feet when the pilot began the landing flare.