Technical
Eccentricity
The orbital element describing the shape of an orbit, from a perfect circle (zero) to increasingly elongated ellipses (approaching one).
Eccentricity measures how much an orbit deviates from a circle. A value of zero is a circular orbit; values between zero and one describe ellipses of increasing elongation; a value of one is a parabolic escape trajectory.
Highly eccentric orbits, such as Molniya-type orbits, spend much of their time near apogee, which is useful for high-latitude coverage. Eccentricity, with the semi-major axis, defines apogee and perigee heights.