As the number of sides grows and approaches infinity, the regular polygon takes on a new shape. It becomes a circle. A circle is a curved line, where each point is the same distance from the center. The circle line or periphery is the outer boundary.
The radius is a straight line from the center to the periphery.
The diameter is a straight line through the center of the circle.
A tangent is a straight line outside the circle, touching only one point of the circle line.