What type of aircraft altitude is most commonly displayed by an altimeter?

An altimeter primarily displays indicated altitude, which is the altitude reading shown on the instrument while the aircraft is in flight. This reading is based on the current atmospheric pressure and is referenced from standard atmospheric conditions. In aviation, indicated altitude is crucial as pilots use it for navigation, ensuring safe clearance from terrain and other aircraft.

Indicated altitude can be affected by changes in atmospheric pressure; therefore, pilots often need to adjust the altimeter setting according to local air pressure for accurate readings. While true altitude (the actual height above sea level), pressure altitude (altitude based on a standard atmospheric pressure setting), and ground elevation (the height of the terrain) are relevant concepts in aviation, indicated altitude is the direct output from the altimeter in its operational state. This makes it fundamental for pilots during flight operations.