An ellipsoid is a three-dimensional geometric shape that resembles a stretched or compressed sphere. It is defined by its three axes, which can be of different lengths, making it wider in one direction and narrower in another. The most common type of ellipsoid is the spheroid, which can be either oblate (flattened at the poles) or prolate (elongated along the axis). 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 helps in accurate mapping and navigation.