In a plane, a circle is the set of all points equidistant from a given point called the center. You can name a circle by its center. Circle is shown.
A radius is a segment that has one endpoint at the center and the other endpoint on the circle. In the circle , is the radius. and are also radii.
A diameter is a segment that contains the center of a circle and has both endpoints on the circle. In the circle , is a diameter.