eccentricity
n.the shape of an orbit that is not a perfect circle. It measures how much an object's path around another is stretched out.
n.a measure of the deviation of an orbit from a perfect circle. In astronomy, it is defined as the ratio of the distance between the foci of the ellipse to the length of its major axis.
“The planet has a high eccentricity, so its orbit is very oval.”
“Astronomers use the eccentricity of a comet's orbit to predict how close it will come to the sun during its next visit.”
“While most planets in the solar system have nearly circular orbits with low eccentricity, the highly elliptical path of Halley's Comet is a classic example of significant orbital eccentricity.”
“A circle has an eccentricity of zero, while any value over one means that an object isn’t orbiting, but rather taking a one-way trip around a gravitational attractor.”
From eccentric + -ity.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.