Here is the meaning of the word, ARC

Arc ( n. ) - A curvature in the shape of a circular arc or an arch; as, the colored arc (the rainbow); the arc of Hadley's quadrant.
Arc ( n. ) - A portion of a curved line; as, the arc of a circle or of an ellipse.
Arc ( n. ) - An arch.
Arc ( n. ) - 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.