An altimeter is an active instrument used to measure the altitude of an object above a fixed level. For example, a laser altimeter can measure height from a spacecraft above an ice-sheet. That measurement, coupled with radial orbit knowledge, will enable determination of the topography.
The traditional altimeter found in most aircraft works in measuring the air pressure from a static port in the airplane. Air pressure falls almost linearly with altitude - about one millibar per 30 feet at typical flying levels. The altimeter is calibrated to show the pressure directly as altitude. The pilot must adjust the altimeter to account for the local air pressure at ground level, which varies with the weather. In pilot's jargon, the regional air pressure at mean sea level is called the QNH, and the pressure which will calibrate his altimeter to show the height above ground at a given airfield is called the QFE of the field.
Other types of altimeter are the radar altimeter that measures the altitude more exactly using the time taken for a radio signal to reflect from the surface back to the aircraft. The radar altimeter is used to measure the exact height during the landing procedure of commercial aircraft.