El-lip-tĭc'i-ty, n.

(-tĭs'ĭ-tĭ), n. [Fr. ellipticité.] Deviation from the form of a circle or sphere; especially, in reference to the figure of the earth, the difference between the equatorial and polar semi-diameters, divided by the equatorial; thus, the ellipticity of the earth is (\frac{1}{299.66}).

Some writers use ellipticity as the ratio of the difference of the two semi-axes to the minor axis, instead of the major. Nichol.