Eccentricity (e) is a number indicating the degree to which an orbit is non-circular, 0 meaning circular, between 0 and 1 being elliptical, 1 being parabolic, and greater than 1 being hyperbolic.
The eccentricity of an elliptical orbit is:
ra - rp e = ——————— ra + rp
Earth's orbit has an eccentricity of 0.0167. The Moon's is 0.0549. Halley's Comet's is 0.97.
Eccentricity can aid in the study of atmospheres of extra-solar planets, i.e., bodies orbiting stars, because the changes in EM radiation from the host star allow observation of the atmosphere at multiple temperatures using light curve spectroscopic observations.