An aneroid barometer calibrated to convert atmospheric pressure into altitude.
American Meteorological Society, Glossary of Meteorology 
Altimeters use standard atmosphere pressure–height relations in converting pressure into altitude. Therefore, the altimeter shows indicated altitude, which may, and frequently does, differ from the actual altitude. An altimeter may be set to measure altitude from an arbitrarily chosen level. It is common practice to use mean sea level; the level of the constant-pressure surface of 29.92 in. of mercury is also used; and, less frequently, the constant-pressure surface of the pressure at airport height. See altimeter setting, pressure altitude. (same source)