eccentricity

The quality of being eccentric or odd; any eccentric behaviour.

The ratio, constant for any particular conic section, of the distance of a point from the focus to its distance from the directrix.

"For an ellipse, the eccentricity is the ratio of the distance from the center to a focus divided by the length of the semi-major axis."

The eccentricity of the conic section (usually an ellipse) defined by the orbit of a given object around a reference object (such as that of a planet around the sun).