- an imaginary surface that coincides with mean sea level in the ocean and its extension through the continents.
- the geometric figure formed by this surface, an ellipsoid flattened at the poles.
- a hypothetical surface that corresponds to mean sea level and extends at the same level under the continents
- the shape of the earth
- The hypothetical surface of the Earth that coincides everywhere with mean sea level and is perpendicular, at every point, to the direction of gravity. The geoid is used as a reference surface for astronomical measurements and for the accurate measurement of elevations on the Earth’s surface.