In mathematics, the eccentricity (sometimes spelled "excentricity"), denoted ε (or, for basic text notation "e"), is a parameter associated with every conic section. It can be thought of as a measure of how much the conic section deviates from being circular. In particular, * The eccentricity of a circle is zero. * The eccentricity of an ellipse which is not a circle is greater than zero but less than 1. * The eccentricity of a parabola is 1. * The eccentricity of a hyperbola is greater than 1.