Which term describes the outer boundary of a circle?

The term that describes the outer boundary of a circle is circumference. The circumference refers to the distance around the circle, essentially forming its perimeter. It can be calculated using the formula (C = 2\pi r) or (C = \pi d), where (r) is the radius and (d) is the diameter of the circle. The circumference is critical in various applications including geometry, engineering, and everyday contexts, as it delineates the circular boundary that separates the circle from the space outside it.

In contrast, surface area pertains to three-dimensional shapes, calculating the total area that the surface of the object occupies, making it irrelevant when discussing a two-dimensional circle. The diameter is a specific measurement that refers to the longest distance across the circle, passing through its center, while the radius indicates the distance from the center to any point on the circumference. Although these terms are related to circles, they do not describe the outer boundary itself.