al·tim·e·ter / alˈtimitər/ (abbr.: alt.) • n. an instrument for determining altitude attained, esp. a barometric or radar device used in an aircraft.

altimeter Instrument for measuring altitude. The simplest type is a form of aneroid barometer. As height increases, air pressure decreases, so the barometer scale can be calibrated to show altitude. Some aircraft have a radar altimeter, which measures the time taken to bounce a radar signal off the ground.