Geometric term for the measure of how much a conic section deviates from being circular.
The eccentricity of an ellipse takes a value between 0 and 1.
When the eccentricity is 0, the conic section is a circle.
orbital eccentricity
Compound of 離心 (りしん, 'off-center') and 率 (りつ, 'rate, ratio').