The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Note: 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]

WordNet (r) 3.0 (2006):

ellipticity
    n 1: the property possessed by a round shape that is flattened
         at the poles; "the oblateness of the planet" [syn:
         oblateness, ellipticity]