# Radius and Diameter

Radius and diameter are the first vocabulary learned by you related to circles. The radius or the diameter determines the size of a circle. A circle is defined to be the set of all points in a plane that are equidistant from a fixed point called the center of the circle.

Radius, chord and diameter are different types of line segments that are defined in relation to a circle.

- A radius of a circle is the line segment joining the center of the circle and any point on the circle. In the above diagram OA is a radius of the circle.
- A chord of a circle is the line segment joining two points on the circle. Segments PQ and BC are both chords of the circle shown.
- A diameter is a chord of the circle which passes through the center of the circle. Chord PQ is a diameter of the circle as it contains the center O of the point.