- A portion of a curved line; as, the arc of a circle or of an
ellipse. - A curvature in the shape of a circular arc or an arch; as, the
colored arc (the rainbow); the arc of Hadley's quadrant. - An arch. - The apparent arc described, above or below the horizon, by the
sun or other celestial body. The diurnal arc is described during the
daytime, the nocturnal arc during the night.