Which instrument is used by pilots to measure altitude?

The instrument used by pilots to measure altitude is an altimeter. An altimeter works by measuring the atmospheric pressure outside the aircraft and translating that pressure into an altitude reading. As an aircraft ascends or descends, the change in air pressure enables the altimeter to indicate the current altitude, allowing pilots to maintain a safe flight level in relation to terrain and air traffic. This is crucial for navigational safety and compliance with air traffic control instructions.

In contrast, other instruments like a tachometer measure the engine's rotational speed, a barometer measures atmospheric pressure specifically without converting it to altitude, and an airspeed indicator shows the speed of the aircraft relative to the surrounding air. Each of these instruments has a distinct purpose and is vital for different aspects of flight, but the altimeter is specifically designed for measuring altitude.