An ellipsoid is a shape like a sphere, but prolate (longer in one or more directions) like a rugby ball, or like an egg. It is the 3 dimensional analogue of an ellipse.
The Earth is shaped slightly like an ellipsoid. Its shape is called an oblate spheroid. An ellipsoid shaped like the Earth, called a reference ellipsoid, is often preferred to the geoid because it makes calculations simpler.
