An arc is a shape that is composed of many points, but is not closed, so has two endpoints. (Ideally, an arc would be composed of an infinite number of points; practically, they are composed of straight lines that are joined by points called nodes.) Examples are rivers, streets, and pathways. From a mathematical perspective, an arc has a length, a beginning and an end, a right and a left. Arcs that have direction are sometimes called routes.