An arc is defined as the portion of the circle defined by two distinct points on the circle. Think of a plate and if you pick two points on the outer edge and connect them via the edge, you will get a curve which is an arc. If you pick two points opposite of each other you will get an arc of pi. If you pick a point that is both the start and the ending point, then you will just trace the entire cricle and this is simply equivalent to the circumference.