Circumference

Circumference, in the ordinary sense, is the curve which encloses a circle, ellipse, or other plane figure, the word perimeter being generally employed if the boundary is made up of straight lines, as with the square, hexagon, or other polygon. The circumference of a circle or ellipse is longer than the perimeter of any ordinary inscribed polygon, and shorter than that of any circumscribed polygon. Strictly speaking, a circle is the curve itself, not the space enclosed by the curve; it is a line, not an area. The length of the circumference of a circle bears a definite ratio to the length of its diameter, this ratio being the same for all circles. It is the same ratio that the area of the circle bears to the square on its radius, or that the area of an ellipse bears to the rectangle contained by its two semi-axes.