Geoid

The equipotential surface (i.e., having the same gravity potential at each point) that best fits the mean sea level (see relative sea level) in the absence of astronomical tides; ocean circulations; hydrological, cryospheric and atmospheric effects; Earth rotation variations and polar motion; nutation and precession; tectonics and other effects such as post-glacial rebound. The geoid is global and extends over continents, oceans and ice sheets, and at present includes the effect of the permanent tides (zero-frequency gravitational effect from the Sun and the Moon). It is the surface of reference for astronomical observations, geodetic levelling, and for ocean, hydrological, glaciological and climate modelling. In practice, there exist various operational definitions of the geoid, depending on the way the time-variable effects mentioned above are modelled.

Definition courtesy of IPCC AR4.