Altimeter is an instrument used to measure the altitude of an aeroplane above a fixed level. Altitude can be determined based on the measurement of Atmospheric Pressure. The greater the altitude, the lower the pressure. When a barometer is supplied with a nonlinear calibration so as to indicate altitude, the instrument is a type of altimeter called a Pressure Altimeter or Barometric Altimeter.

Altimeter setting is the value of the atmospheric pressure used to adjust the scale of a pressure altimeter so that it indicates the height of an aircraft above a known reference surface. This reference can be the Mean Sea Level pressure (QNH), the pressure at a nearby surface airport (QFE), or the "Standard Pressure Level" of 1,013.25 hectopascals (29.92 inches of mercury) which gives Pressure Altitude and is used to maintain one of the standard Flight Levels. The setting of a sensitive pressure altimeter is shown in the Kollsman window.

The QNH altimeter setting is one of the data included in METAR messages. An alternative setting is QFE or QNE.

QNH is the barometric altimeter setting that causes an altimeter to read aircraft elevation Above Mean Sea Level - altitude (AMSL) in International Standard Atmosphere (ISA) temperature conditions in the vicinity of the airfield that reported the QNH value.

QFE is the barometric altimeter setting that causes an altimeter to read zero when at the reference datum of a particular airfield (in practice, the reference datum is either an airfield center or a runway threshold). In ISA temperature conditions the altimeter will read the height above the airfield/runway in the vicinity of the airfield.

Standard Setting refers to the altimeter being set to the standard pressure of 1013.25 hPa. It is the setting that causes an altimeter to read the aircraft's Fight Level (FL). Flight levels are given in hundreds of feet (for example: FL100 = 10.000 ft). Atmospheric pressure changes over time and position. Thus, the Flight Level is not "straight", because it has a different altitude (elevation above the mean sea level).

The purpose of the altimeter subscale is to reference the altimeter reading to a predetermined level such as Mean Sea Level, aerodrome level or Pressure Level 1013.25 hPa.