1. (Geom.) Any part of a curved line. 2. (Arch.) (a) Usually a curved member made up of separate wedge-shaped solids, with the joints between them disposed in the direction of the radii of the curve; used to support the wall or other weight above an opening. In this sense arches are segmental, round (i. e., semicircular), or pointed. (b) A flat arch is a member constru...