Arc \Arc\, n. [F. arc, L. arcus bow, arc. See {Arch}, n.] 1. (Geom.) A portion of a curved line; as, the arc of a circle or of an ellipse. [1913 Webster] 2. A curvature in the shape of a circular arc or an arch; as, the colored arc (the rainbow); the arc of Hadley's quadrant. [1913 Webster] 3. An arch. [Obs.] [1913 Webster] Statues and trophies, and triumphal arcs. --Milton. [1913 Webster] 4. 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. [1913 Webster] {Electric arc}, {Voltaic arc}. See under {Voltaic}. [1913 Webster]