A circle of radius a compressed to an ellipse.
Ellipticity, also known as oblateness or flattening, is a measure of the amount by which an object, such as a planet or a galaxy, deviates from a perfect sphere.
It is found by subtracting the polar diameter from the equatorial diameter, then dividing by the equatorial diameter.
Ellipticity is typically an indication of how fast a body is rotating. The ellipticity of Earth is about 1/299. The ellipticity of elliptical galaxies increases in going from type E0 to E7.