Is the level surface, equipotential in the gravitational field, which adopts an irregular threedimensional spheroid. Due to the dependency on the mass distribution inside the Earth, it is impossible to represent it mathematically. The mathematical shape used to do it is the ellipsoid that approximately fits better the Earth shape. The geoid is coincident with the mean ocean surface when they are quiet, virtually extended for the continents land.
ORIGIN: OSGeo Glossary (last updated: 2019-10-29)