# 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.

Diameter is an elementary-school-level concept. It is listed in the California State Standards for Grade 4.

### Prerequisites

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

Sphere: | A sphere is the set of all points in three-dimensional space that are located at a fixed distance from a given point. |