M.a.s.l.

Metres above mean sea level (MAMSL) or simply metres above sea level (MASL or m a.s.l.) is a standard metric measurement in metres of the elevation or altitude of a location in reference to a historic mean sea level. Mean sea levels are affected by climate change and other factors and change over time. For this and other reasons, recorded measurements of elevation above sea level might differ from the actual elevation of a given location over sea level at a given moment.

Metres above sea level is the standard measurement of the elevation or altitude of:

The elevation or altitude in metres above sea level of a location, object, or point can be determined in a number of ways. The most common include:

Accurate measurement of historical mean sea levels is complex. Land mass subsidence (as occurs naturally in some islands) can give the appearance of rising sea levels. Conversely, markings on land masses that are uplifted due to geological processes can suggest a lowering of mean sea level.

Feet above sea level is the most common analogue for metres above sea level in the American measurement system.

Metres above sea level is commonly abbreviated mamsl or MAMSL, based on the abbreviation AMSL for above mean sea level. Other abbreviations are m.a.s.l. and MASL.

...
Wikipedia