Definify.com
Webster 1913 Edition
Ellipticity
Elˊlip-tic′i-ty
,Noun.
[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.
☞ Some writers use ellipticity as the ratio of the difference of the two semiaxes to the minor axis, instead of the major.
Nichol.