A circle doesn't really have an edge, per se. But I think what you are referring to is a tangent.
A tangent is a line that touches the perimeter of the circle at only one point.
(see related link)
A tangent line.
diameter.
If you draw a line from the center of a circle to the edge of a circle, you have drawn the radius of the circle. If you draw a line from the edge of a circle through the center of the circle and on to the edge on the opposite side, you have drawn the diameter of a circle. The radius of a circle is one half the diameter of a circle.
A straight line from the center of a circle to the outer edge is called a radius.
Tangent
Is a tangent
A tangent line.
diameter
A tangent
A straight line touching the circumference of circle at one point is a tangent
A chord
diameter.
If you draw a line from the center of a circle to the edge of a circle, you have drawn the radius of the circle. If you draw a line from the edge of a circle through the center of the circle and on to the edge on the opposite side, you have drawn the diameter of a circle. The radius of a circle is one half the diameter of a circle.
A straight line from the center of a circle to the outer edge is called a radius.
Tangent
tangent
The line from the middle of a circle to the edge is called a radius. It measures the distance from the center of the circle to any point on its circumference. The radius is half the length of the diameter, which is the line that passes through the center and connects two points on the edge of the circle.