An altimeter is an instrument in an aircraft that shows the height of the aircraft above the ground. an instrument in an aircraft that tells you how high you are (altus ( ALTITUDE) + -meter). Instrument that measures the altitude of the land surface or of any object, such as an airplane. The mechanical pressure altimeter measures atmospheric pressure relative to sea level through a series of bellows, gears, and springs, which move pointers on a dial. Radio altimeters measure the distance of an aircraft above the ground rather than above sea level by indicating the time a pulse of radio energy takes to travel from the aircraft to the ground and back; they are used in automatic navigation and blind-landing systems