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|mean sea level|
The average depth of the water exposed to the ocean. It can be thought of as the half way point between mean high water and mean low water, although there must be more specific definitions due to the effect of atmospheric pressure. See The Barometer Handbook, under Inverse Barometer Effect.
(This is the level (MSL) that is slowly increasing with time as the ice caps melt due to global warming.)
some topo maps and other land based maps reference elevations to MSL, but nautical charts use MHW.