eccentricity/eccentric: An ellipse can be very close to the shape of a circle, or it can be very long and narrow, or it can be anywhere in between. The extent that an ellipse differs from a circular shape is called its eccentricity. A long and narrow ellipse is said to be very eccentric. An almost circular ellipse has little eccentricity. Orbits are often described in terms of their eccentricity.