An ellipsoid is a three-dimensional geometric shape that resembles a stretched or compressed sphere. It is defined mathematically by its three axes, which can be of different lengths. The most common type of ellipsoid is the spheroid, where two axes are equal, resulting in a shape that is either flattened at the poles or elongated along the equator. Ellipsoids are important in various fields, including geodesy, where they are used to model the shape of the Earth. The Earth's shape is often approximated as an oblate ellipsoid, which accounts for its slight flattening at the poles and bulging at the equator due to rotation.