What feature is unique about a circle compared to polygons?

A circle is unique among shapes because it is defined by its radius, which is the distance from the center of the circle to any point on its circumference. This property distinguishes it from polygons, which have straight sides and defined angles. Unlike polygons that rely on vertices and line segments to form a closed figure, a circle is a continuous curve with no corners or straight edges, emphasizing the importance of the radius in its definition. The radius helps determine the size of the circle and is essential for calculating its area and circumference, further illustrating the unique characteristics of circular geometry in contrast to the properties of polygons.