An arc of a circle is the portion of the circle between two given points on the circle. More generally, an arc is a portion of a smooth curve joining two points.
The measure of a circular arc on circle is defined to be the measure of the central angle which has the arc on its interior. The length of an arc can be calculated by the formula , where is the radius of the circle and is the measure of the arc, in radians. Thus, in particular, the circumference of a circle is given by .
Arc is also used as a prefix. For each of the standard trigonometric functions, the arc-function is (one of) the corresponding inverse function. For example, the arcsine is the inverse of the sine.