eccentricity

Eccentricity is a single number, written e, that captures the shape of an ellipse. At e = 0 the ellipse is a perfect circle. As e grows toward 1 the ellipse stretches, becoming longer and thinner. At e = 1 it degenerates into a parabola; beyond that, the curve opens up into a hyperbola.

Earth's orbit has an eccentricity of about 0.017 — almost circular. Mercury, the most eccentric of the classical planets, has an eccentricity of about 0.21. Many comets have eccentricities close to 1, on long elongated paths that swing in past the sun and back out to the outer solar system.