Definition
An aneroid instrument that measures the height of the aircraft above a selected pressure reference by sensing the surrounding atmospheric pressure and converting it into an altitude reading in feet. The pilot sets the reference pressure in the altimeter's Kollsman window so the instrument displays altitude relative to that reference (typically the local altimeter setting, which produces height above mean sea level).
Plain English
A cockpit gauge that figures out how high you are by measuring how much air pressure is pushing on it. Higher up means less pressure, so the gauge converts that pressure into a height reading you can read in feet.
Context Anchor
Seen in altimeter-setting procedures, instrument flying, and any discussion of local barometric pressure settings such as QNH.
Derivation
From Latin altus (high) and the Greek-rooted metron (measure) -- literally a 'height measurer.' The 'pressure' part highlights how it does the measuring: through air pressure rather than radio signals or GPS.
Why Pilots Care
Correct use ensures accurate altitude for terrain clearance, airspace compliance, and safe instrument flight.
Analogy
A pressure altimeter is like a weather barometer with the scale marked in feet instead of weather changes. It senses air pressure, then displays that pressure as an altitude.
Grounding Statement
As an aircraft climbs, outside air pressure drops, and the pressure altimeter interprets that lower pressure as a higher altitude.
Intuition Check
Do not assume a pressure altimeter directly measures true height above the ground. It measures air pressure and turns that pressure into an altitude reading based on the setting in the instrument.
Example Sentence 1
Before takeoff, the pilot set the local altimeter setting in the pressure altimeter so it read the published field elevation.
Example Sentence 2
During the approach the pressure altimeter showed level flight at the assigned altitude.