The figure the Earth would have if it were entirely covered by water at mean sea level. The geoid is approximately an ellipsoid, but there are departures from it caused by the gravitational attraction of mountains, as well as differences in density within the Earth. The concept of the geoid is now being applied to other planetary bodies as well. In such cases, an arbitrary datum is used in place of sea level, such as the planet’s mean radius or, for gaseous worlds, the altitude at which a given atmospheric pressure is found.