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.
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A tangent line.
diameter.
A straight line from the center of a circle to the outer edge is called a radius.
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.
Tangent