A line segment joining the centre of the circle to any point on the circle is called  it’s

A line segment joining the centre of the circle to any point on the circle is called radius of the circle

                                         

$\text{'}O\text{'}$  is called the centre of the circle and  $OA$ is called the radius of the circle it is denoted $\text{'}r\text{'}$ . A circle has unlimited number of radii.