From AMS Glossary
The altitude that corresponds to a given value of atmospheric pressure according to the ICAO standard atmosphere.
For aircraft flying above 18 000 feet MSL, it is the indicated altitude of a pressure altimeter at an altimeter setting of 1013.2 mb (29.92 in. of mercury); therefore, it is the indicated altitude above the 1013.2-mb constant-pressure surface. Aircraft flying below 18 000 feet MSL use the current altimeter setting as measured at the nearest airport and reported by air traffic control.