Which instrument is used to measure the aircraft's altitude?

The instrument used to measure an aircraft's altitude is the altimeter. An altimeter functions by measuring the atmospheric pressure, as atmospheric pressure changes with altitude. Specifically, as an aircraft ascends, the air pressure decreases, and the altimeter translates this drop in pressure into a corresponding altitude reading. Most altimeters are calibrated to indicate altitude above sea level based on the standard atmosphere conditions.

In terms of instrument types, a variometer is primarily used to measure the rate of climb or descent rather than altitude itself, which is a different function. A barometer also measures atmospheric pressure but is usually not employed for altitude readings in aviation contexts. Lastly, a speedometer is designed to measure airspeed and has no relation to altitude measurement. Understanding these distinctions helps clarify the specific role of each instrument in aviation terminology and operations.