Source : Webster's Revised Unabridged Dictionary (1913)

Ellipticity \El`lip*tic"i*ty\, n. [Cf. F. ellipticit['e].]
   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.

   Note: Some writers use ellipticity as the ratio of the
         difference of the two semiaxes to the minor axis,
         instead of the major. --Nichol.

Source : WordNet®

ellipticity
     n : the property possessed by a rounded shape that is flattened
         at the poles; "the oblateness of the planet" [syn: {oblateness}]