What does an Altimeter measure?

An altimeter measures the altitude based on changes in air pressure. As an aircraft ascends or descends, the atmospheric pressure changes, and the altimeter senses these changes to provide a relative altitude reading above sea level. The principle behind an altimeter’s operation is that as altitude increases, the air pressure decreases. By calibrating to a standard atmospheric pressure at sea level and referencing local pressure settings, the altimeter can accurately determine how high the aircraft is flying.

The other options represent measurements that are not the function of an altimeter. For instance, while the speed of the aircraft is measured by a separate instrument called aairspeed indicator, the direction of flight is indicated by a heading indicator. The temperature inside the cockpit can be monitored, but it is unrelated to altitude measurements. Thus, the only correct function of an altimeter is to measure altitude through air pressure variations.