Which instrument primarily indicates the aircraft's altitude?

The instrument that primarily indicates the aircraft's altitude is the altimeter. The altimeter functions by measuring the atmospheric pressure outside the aircraft. As altitude increases, the air pressure decreases; thus, the altimeter is calibrated to convert these pressure changes into a vertical height above sea level. Pilots use this crucial information to maintain their desired flight level, navigate effectively, and ensure safety during ascent, cruising, and descent.

While other instruments like the airspeed indicator provide essential flight data, they do not indicate altitude. The artificial horizon helps the pilot maintain situational awareness in terms of the aircraft's orientation but does not provide altitude information. The vertical speed indicator indicates how quickly the aircraft is ascending or descending but does not directly show the current altitude itself. Hence, the altimeter is the specific instrument dedicated to providing altitude readings.