What primary instrument indicates an aircraft's altitude?

The primary instrument that indicates an aircraft's altitude is the altimeter. This device measures the atmospheric pressure outside the aircraft and translates it into altitude above a reference point, typically sea level. The altimeter operates on the principle that air pressure decreases as altitude increases; therefore, it can provide an accurate reading of how high the aircraft is flying above sea level. Pilots rely on the altimeter not only for navigation but also for maintaining safe separation from terrain and other aircraft. Understanding and correctly interpreting the altimeter reading is crucial for flight safety and adherence to altitude regulations.