mean sea level
Technical term used in geography, oceanography, and surveying. Refers to the average height of the sea surface over a long period, used as a reference for elevation.
Mean sea level is used as the reference for elevation.
The contour lines on this map indicate height above mean sea level.
Compound of 平均 (mean, average), 海水 (seawater), and 面 (surface). A straightforward technical term with no obscure historical derivation.