I'm sure you have seen or used a compass already. The pivot point is the needle part. It is called as such because it is the point where the compass turns so that the movie end of the other leg of the compass can make arcs.
As you create a circle using a compass, that same distance between the two legs of the compass is the radius of the circle. By definition, a circle is a round two-dimensional figure that is composed of a series of points that is equidistant to a fixed point inside the circle. That point is called the center of the circle represented by the pivot or needle part. The equal distance of all the points along the circumference is the radius.
