# Circle

A circle is the set of points in a plane that are equidistant from a given center point.

Circle is an elementary-school-level concept. It is listed in the California State Standards for Grade 1.

### Examples

Unit Circle: | The unit circle is a circle of radius one. |

### Prerequisites

Diameter: | (1) In plane geometry, the diameter is the straight-line distance between two points opposite one another across the center of a circle. (2) In solid geometry, the diameter is the straight-line distance between two antipodal points on a sphere. |

Radius: | The radius of a circle is the distance from its center to its circumference or from the center of a sphere to its surface. The radius is equal to half the diameter. |