Arc

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