What instrument is used to measure altitude in an aircraft?

The instrument used to measure altitude in an aircraft is the altimeter. An altimeter functions by measuring the atmospheric pressure outside the aircraft. As an aircraft ascends or descends, the air pressure changes, and this variation in pressure is translated into altitude through a calibrated scale. Most altimeters are barometric altimeters, which depend on the principle that atmospheric pressure decreases as altitude increases.

While a barometer is related because it also measures atmospheric pressure, it is not typically used for altitude measurement in aviation applications. The speedometer measures the speed of the aircraft, and the heading indicator provides information about the aircraft's direction, but neither of these instruments is involved in altitude determination. Hence, the altimeter is specifically designed for this purpose, making it the correct answer in this context.