Mean Sea Level (MSL) is the datum for measurement of elevation and altitude that is the average level of the sea’s surface, calculated as the arithmetical mean of hourly tide levels taken over an extended period and used as the standard for determining terrestrial and atmospheric elevations and sea depths. A common and relatively straightforward MSL (Mean Sea Level) standard is the midpoint between a mean low and mean high tide at a particular location.
Reference Definition by Ametsoc.org: In the United States, mean sea level is defined as the mean height of the surface of the sea for all stages of the tide over a 19-year period. Selected values of mean sea level serve as the sea level datum for all elevation surveys in the United States. In meteorology, mean sea level is used as the reference surface for all altitudes in upper-atmospheric work; in aviation it is the level above which altitude is measured by a pressure altimeter. Along with mean high water, mean low water, and mean lower low water, mean sea level is a type of tidal datum.
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