From AMS Glossary
The particular geopotential surface that most nearly coincides with the mean level of the oceans of the earth.
For mapping purposes it is customary to use an ellipsoid of revolution as an adequate and convenient approximation to the geoid. The dimensions and orientation of the assumed ellipsoid may represent an attempt to find the ellipsoid that most nearly fits the geoid as a whole, or they may represent an attempt to fit only a particular part of the geoid without regard to the remainder of it. When mention is made of the dimensions of the earth, reference is usually made to the dimensions of the ellipsoid most nearly representing the geoid as a whole. The actual geoid can depart from a best-fitting sphere in places by as much as 100 m.