ə̇ˌlipˈtisəd.ē, eˌ-, ēˌ-, ˌe(ˌ)- noun
( -es )
1. : deviation of an ellipse or a spheroid from the form of a circle or a sphere
2. : the difference between the equatorial and polar semidiameters, divided by the equatorial or occasionally by the polar — used especially in ref. to the figure of the earth
the ellipticity of the earth is approximately 1/300