\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.