ra‧di‧us plural radii [countable]
the distance from the centre to the edge of a circle, or a line drawn from the centre to the edge [↪ diameter]
an area that covers a particular distance in all directions from a central point:
The shock of the explosion was felt over a radius of forty miles.
within a 10-mile/200-metre etc radius
There are more than a dozen golf courses within a 15-mile radius of St Andrews.
the outer bone of the lower part of your arm