the definition of arc
An arc is a portion of the circumference of a circle. In the figure above, the arc is the blue part of the circle. Strictly speaking, an arc could be a portion of some other curved shape, such as an ellipse, but it almost always refers to a circle. To avoid all possible mistake, it is sometimes called a circular arc. A straight line is drawn between the end points of the arc would be a chord of the circle. If the arc length is exactly half the circle, this called a semicircular arc. See Semicircle definition.