In mathematics, **eccentricity** 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 is greater than zero and smaller than 1
- The eccentricity of a parabola is 1.
- The eccentricity of a hyperbola is greater than 1.
- The eccentricity of a straight line is infinity.

In popular usage,

Many of history's most brilliant minds have displayed many unusual behaviors and habits, and eccentricity is sometimes thought to be a sign of genius. However, many eccentrics are crankss, rather than geniuses.

**Extravagance** is a kind of eccentricity, related to abundance and wastefulness.