A radius of a circle is any straight line drawn from the
center of the circle to a point on its circumference. The
phrase the radius of a circle is often taken to mean the length
of the radius of the circle.

So for example, if asked to find
the radius of a bicycle wheel it is the length of its radius
which is required. The radius of a circle is the distance
from the centre of the circle to its edge. The radius of a
regular polygon is the distance from the centre of the polygon
to any one of its vertices. The plural of radius is radii.

Example of Radius of the Circle: