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.

The elongation, or flattening, of an ellipse, expressed as the decimal fraction by which the minor, or shortest, axis is less than the major, or longest, axis. If the minor axis were 70% less than the major axis, the elllipticity would be 0.7. Often used to express the shape of actual physical structures which are approximately oblate spheroids, such as planets, and elliptical galaxies. Elliptical galaxies are actually classified according to their apparent ellipticity, rounded off to the nearest 10%. Thus, an ellliptical galaxy which appears to have an ellipticity of about 60% would be classified as an E6.

the property possessed by a rounded shape that is flattened at the poles; "the oblateness of the planet"

Describes the fact that the core or cladding may be elliptical rather than circular. EM: Abbreviation for electromagnetic.