Name one primary flight instrument indicated by the aircraft's altimeter.

The altimeter is a crucial primary flight instrument specifically designed to provide an indication of an aircraft's elevation or altitude above a predetermined reference point, typically sea level. It works by measuring the atmospheric pressure outside the aircraft; as altitude increases, air pressure decreases. The instrument is calibrated to convert this pressure data into altitude readings, enabling pilots to gauge how high the aircraft is flying.

Understanding altitude is vital for several reasons. It ensures safe clearance from terrain and obstacles, informs decisions related to flight maneuvers, and aids in air traffic control communication. Therefore, the indication provided by the altimeter directly relates to the vertical position of the aircraft, making elevation or altitude the most accurate response to this question.