Mean Sea Level. The average height of the surface of the sea for all stages of tide; used as a reference for elevations, and differentiated from AGL. Found on http://www.aerofiles.com/glossary.html
Mean sea level. Used by pilots when defining altitude, MSL refers to feet above sea level as opposed to above the ground. Pilots always use MSL when referring to altitude Found on http://www.encyclo.co.uk/local/20082
Mean Sea Level. The average height of the surface of the sea for all stages of tide; used as a reference for elevations, and differentiated from AGL. Found on https://www.encyclo.co.uk/local/22323