The geoid is the equipotential surface of the Earth's gravity field that best approximates mean sea level. In simpler terms, it is a hypothetical surface that represents the Earth's shape if the oceans were at rest and in equilibrium under the gravitational pull of the Earth. It is often described as the "true" shape of the Earth, as it takes into account the irregularities in the Earth's gravity field caused by variations in the planet's density and mass distribution. The geoid is an important concept in geodesy, the science of measuring the shape and size of the Earth. It is used as a reference surface for measuring heights and depths, as well as for determining the location and motion of satellites and other objects in space. GPS receivers, for example, measure the distance between the receiver and a number of satellites to determine the receiver's position on the Earth's surface relative to the geoid.
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