Geoid

From DictionGloss

Geoid - The equipotential (level) surface of the earth's gravity field that, on average, coincides with the mean sea level in the open undisturbed ocean. In practical terms, the geoid is the imaginary surface where the oceans would seek mean sea level if allowed to continue into all land areas so as to encircle the earth. The geoid undulates up and down with local variations in the mass and density of the earth. The local direction of gravity is always perpendicular to the geoid.

Geoid height - The height above the geoid is often called elevation above mean sea level.