*ellipticity*

n. [ Cf. F. ellipticité. ] 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 1/29966. [ 1913 Webster ]

☞ Some writers use ellipticity as the ratio of the difference of the two semiaxes to the minor axis, instead of the major. Nichol. [ 1913 Webster ]