What is the radius of a circle?

The concept of “radius” is associated with concepts such as “circle”, “center of a circle”, “chord” and “diameter”, so to define what is “radius”, you need to know what the above terms are.
We put a conditional point O on the plane, measure from it, for example, 5 cm in all possible directions and connect all the points obtained. These points will represent a circle, and the starting point will be its center.
On this circle, mark two arbitrary points and connect them with a segment: this segment is called a chord, and the segment that passes through point O is the diameter (d) of the circle and point O is divided in half.
The segments into which the diameter is divided are called radii.
From all this it follows that the radius is the distance from the center of the circle to any of the points on the circle itself. Its length is:
r = d / 2.