Eccentricity of an Orbit

an element of the orbit of a celestial body characterizing the orbit’s shape. Depending on the value of the eccentricity e, the orbit may be elliptical (e < 1), parabolic (e = 1), or hyperbolic (e > 1). The concept of angle of eccentricity φ, based on the relation e = sin φ, is sometimes introduced instead of the eccentricity for elliptical orbits.