Orbital eccentricity

The orbits of celestial objects are all conic sections, and their orbital eccentricity is a measure of how stretched they are.

The circle is a conic section, and circular orbits, if they existed, would have an orbital eccentricity of zero.

The other three conic sections are ellipses, the parabola, and hyperbolas. Objects with elliptical orbits that are close to circular have low eccentricity. Object with orbits that resemble ovals have higher eccentricity.

The parabola is a special case. Parabolas have an orbital eccentricity of one. If an object was created, at rest to Sol, our sun, at the edge of the Universe, it would fall into the Solar system in a parabola, speeding up as it came closer, then swinging around Sol, slowing down as it got farther away.

In the early 21st century Astronomers have detected comets, falling towards Sol, from outside the Solar system. Since they already had some velocity, they are traveling faster than the Solar system's escape velocity, as they approach Sol, and that velocity carries them back into interstellar space. They have what are called "hyperbolic orbits", and their orbital eccentricity is greater than one