An Altimeter is a device which indicates the altitude. Altimeters are widely used in aviation, in mountaineering and in skydiving sports. 
Altimeters work on the principle that the atmospheric pressure drops with altitude. As the height above sea level increases, the pressure drops. The Altimeter is basically a pressure measuring device which is calibrated in terms of distance above sea level.
Altimeters are among the most common instruments in aviation. They are also used by mountaineers to calculate their altitude.
Hand held Altimeters are worn by skydivers.