Radius
The radius is the straight-line distance from the center of a circle or sphere to any point on its outer edge or surface. It is exactly half the length of the diameter.
Example
If a circle has a diameter $d$ of $10$ cm, its radius $r$ is half of the diameter, calculated as $r = \frac{d}{2} = \frac{10}{2} = 5$ cm. The area of this circle is $A = \pi r^2 = \pi (5)^2 = 25\pi$ cm$^2$.