e

The Eccentricity (e) defines how oval the satellite's orbit is. It is mathematically defined as the ratio of the orbit's focus distance (c) to the orbit's semi-major axis (a).

e = c / a

where e = the orbit eccentricity;
           c = the focus distance; and
           a = the satellite orbit's semi-major axis.

The eccentricity of a satellite orbit is a unitless value that lies between 0 (circular orbit) and 1 (parabolic orbit).