Circle: A circle is the locus of a point which moves in a plane in such a way that its distance from a given fixed point is always constant. The fixed point is called the center and the constant distance is called the radius (plural: radii) of the circle. A circle is usually named by its center point. The figure below shows circle O, which can be written as ⊙O but is rarely used.
Note: An important point to note is that the line segment joining the center and any point on the circle is called its radius. That is, radius is used in two senses: (i) in the sense of a line segment and (ii) in the sense of its length.