El`lip*tic"i*ty (?), 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 &frac1x29966;.

&fist; Some writers use ellipticity as the ratio of the difference of the two semiaxes to the minor axis, instead of the major. Nichol.