A level of constant atmospheric pressure related to a reference datum of 29.92 inches of mercury. Each is stated in three digits that represent hundreds of feet. For example, flight level (FL) 250 represents a barometric altimeter indication of 25,000 feet; FL 255, an indication of 25,500 feet.

A surface of constant atmospheric pressure which is related to a specific pressure datum, 1013.2 hPa (1013.2 mb), and is separated from other such surfaces by specific pressure intervals.

• Note 1: A pressure type altimeter calibrated in accordance with the standard atmosphere:
  1. When set to a QNH altimeter setting, will indicate altitude;
  2. When set to a QFE altimeter setting, will indicate height above the QFE reference datum; and
  3. When set to a pressure of 1013.2 hPa (1013.2 mb), may be used to indicate flight levels.
• Note 2: The terms `height' and `altitude,' used in Note 1 above, indicate altimetric rather than geometric heights and altitudes.

Historically, altitude has been measured using a pressure altimeter, which is essentially a calibrated barometer. An altimeter measures air pressure, which decreases with increasing altitude following the barometric formula, and from the surrounding's pressure calculates and displays the corresponding altitude.

To display altitude above sea level, a pilot must recalibrate the altimeter according to the local air pressure at sea level, to take into account natural variation of pressure over time and in different regions. If this is not done, two aircraft could be flying at the same altitude even though their altimeters appear to show that they are at considerably different altitudes.

Flight levels solve this problem by defining altitudes based on a standard air pressure at sea-level. All aircraft operating on flight levels calibrate to this setting regardless of the actual sea level pressure.

Flight levels are described by a number, which is this nominal altitude ("pressure altitude") in feet, divided by 100, while being a multiple of 500 ft, therefore always ending on 0 or 5. Therefore an apparent altitude of, for example, 32,000 feet is referred to as "flight level 320". To avoid collisions between two aircraft due to their being at the same altitude, their "real" altitudes (compared to ground level, for example) are not important; it is the difference in altitudes that determines whether they might collide. This difference can be determined from the air pressure at each craft and does not require knowledge of the local air pressure on the ground.

Flight levels are usually designated in writing as FLxxx, where xxx is a one- to three-digit number indicating the pressure altitude in units of 100 feet. In radio communications, FL290 would be pronounced as "flight level two niner zero", in most jurisdictions. In some states, levels ending with 00 are read as "hundred": FL200 is pronounced as "flight level two hundred". The phrase "flight level" makes it clear that this refers to the standardized pressure altitude.