What Are Radii?

A circle's radius reaches from its center point to the circumference

The radius is kind of the basic unit of a circle: its length is the distance from the center of a circle to any point on its circumference and it forms the basis for the radian.

You can use the length of a circle's radius to find its circumference ($C = 2\pi r$), its area ($A = \pi r^2$), and its diameter ($d = 2r$).

In the image below, the red line segment labeled $r$ is the circle's radius. All the grey line segments are also radii, too, since the radius can connect the center with any point.