What does an altimeter measure?

An altimeter is an instrument used to measure the altitude of an aircraft above a specific reference point, typically sea level. It operates primarily by measuring the atmospheric pressure outside the aircraft. As an aircraft ascends, the air pressure decreases; the altimeter translates this change in pressure into altitude information. This is critical for pilots to maintain proper flight levels, avoid terrain, and comply with air traffic control instructions.

The other options refer to different instruments or parameters: speed relative to the ground is measured with a groundspeed indicator or airspeed indicator, engine performance metrics are tracked by engine instruments, and cockpit air temperature is measured with a temperature gauge. Each of these serves a unique role in flight operation and safety, but none specifically provide altitude data like the altimeter does.