Definition: The distance from the center of a regular polygon to any
vertex .

Try this
Adjust the polygon below by dragging any orange dot, or alter the number of sides. Note the behavior of the polygon's radius.

The radius of a regular polygon is the distance from the center to any
vertex.
It will be the same for any vertex.
The radius is also the radius of the polygon's
circumcircle, which is the circle that passes through every vertex.
In this role, it is sometimes called the circumradius.

Irregular polygons are not usually thought of as having a center or radius.

Radius given the length of a side

By definition, all sides of a regular polygon are equal in length.
If you know the length of one of the sides, the radius is given by the formula: