True shape of the Earth, which deviates from a perfect sphere because of a slight bulge at the equator.

spheroid representation of the Earth that takes into account mean sea level and gravity.

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 [..]

The surface within or around the earth that is everywhere perpendicular to the direction of gravity and coincides with mean sea level in the oceans at rest. Sea level of a moving ocean has topography, [..]

An ellipsoid with a highly irregular surface used to describe the shape of the earth.

[geodesy] A geoid is a measurement-based model of the shape of the Earth primarily used as a basis of assigning terrain height. A geoid works by calculating what height the oceans would settle to when [..]

 the hypothetical surface of the Earth that coincides everywhere with the mean sea level

Figure of the Earth visualized as a mean sea level surface extended continuously through the continents. It is a theoretically continuous surface that is perpendicular at every point to the direction [..]

The shape of the sea surface assuming a complete absence of perturbing forces (tides, wind, currents, etc.). The geoid reflects the Earth's gravitational field (it is an equipotential surface) and varies in height by as much as 100 meters over distances of several thousand kilometers due to uneven mass distribution within the planet's cru [..]

The shape of the earth if it were considered as a sea level surface extended continuously through the continents. The geoid is an equipotential surface coincident with mean sea level to which at every [..]

An imaginary surface which is everywhere perpendicular to the plumb line, and which on average coincides with Mean Sea Level in the open ocean. Its shape approximates to that of a spheroid, but it is [..]

The shape of the sea surface assuming a complete absence of perturbing forces (tides, wind, currents, etc.). The geoid

The equipotential surface of the Earths gravity field which best fits, in the least squares sense, mean sea level.

A surface of constant gravitational potential around the Earth--an averaged surface perpendicular to the force of gravity.

The particular equipotential surface that coincides with mean sea level and that may be imagined to extend through the continents. This surface is everywhere perpendicular to the force of gravity.  

an equipotential surface that would represent the mean sea or ocean surface of the Earth if the oceans and atmosphere were in equilibrium and at rest relative to the rotating Earth and an extension of [..]

A surface of constant gravitational potential at zero elevation.

A geoid is an equipotential surface which approximately coincides with the mean ocean surface. It is a eference system for gravity measurements.