A surface of constant atmospheric pressure which is related to a specific pressure datum, 1 013.2 hectopascals (hPa), and is separated from other such surfaces by specific pressure intervals.
ICAO, Official term and definition, AN 2 Rules of the Air (2005)
Note 1.– A pressure type altimeter calibrated in accordance with the Standard Atmosphere:
a. when set to a QNH altimeter setting, will indicate altitude;
b. when set QFE altimeter setting, will indicate height above the QFE reference datum;
c. when set to a pressure of 1 013.2 hPa, 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 altitude.
Lexicon Release 2015.2