An ellipsoid is a three-dimensional geometric shape that resembles a stretched or compressed sphere. It is defined by three axes of different lengths, which determine its overall shape. The most common type of ellipsoid is the spheroid, where two axes are equal, resulting in a shape similar to a flattened or elongated sphere. Ellipsoids are important in various fields, including geodesy, where they are used to model the Earth's shape. The Earth's surface is not a perfect sphere but rather an oblate spheroid, meaning it is slightly flattened at the poles and bulging at the equator, which can be accurately represented by an ellipsoid.