What does "ellipticity" mean?

Ellipticity: Deviation of an ellipse or a spheroid from the form of a circle or a sphere; especially, in reference to the figure of the earth, the difference between the equatorial and polar semidiameters, divided by the equatorial; thus, the ellipticity of the earth is Note: Some writers use ellipticity as the ratio of the difference of the two semiaxes to the minor axis, instead of the major. Nichol.