An ellipsoid is a three-dimensional geometric shape that resembles a stretched or compressed sphere. It is defined by three semi-axis lengths, which determine its dimensions along the x, y, and z axes. The mathematical equation of an ellipsoid can be expressed as ((x/a)^2 + (y/b)^2 + (z/c)^2 = 1), where (a), (b), and (c) are the lengths of the semi-axes. Ellipsoids are commonly used in applications such as geodesy and satellite positioning to model the Earth's shape.