What instrument measures aircraft altitude?

The instrument that measures aircraft altitude is known as the altimeter. An altimeter works by utilizing atmospheric pressure to determine the altitude of an aircraft above a reference level, typically mean sea level. As an aircraft ascends or descends, the air pressure changes, and the altimeter adjusts its reading accordingly.

In contrast, while a barometer also measures atmospheric pressure, it is primarily used for weather forecasting and does not directly indicate altitude in a way that is practical for aviation. An altitude indicator provides information derived from other systems and is sometimes used in conjunction with an altimeter but does not measure altitude directly on its own. A compensator is generally associated with systems that adjust for certain variables but is not specifically an altitude measuring instrument. The altimeter is the designated instrument for accurately providing altitude information to pilots, thus playing a crucial role in navigation and safety during flight.