Flight Level [ICAO]

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.

Source: Pilot Contoller Glossary (PCG)