Source : Webster's Revised Unabridged Dictionary (1913)

Arch \Arch\, n. [F. arche, fr. LL. arca, for arcus. See {Arc}.]
   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 constructed of stones cut into
          wedges or other shapes so as to support each other
          without rising in a curve.

   Note: Scientifically considered, the arch is a means of
         spanning an opening by resolving vertical pressure into
         horizontal or diagonal thrust.

   3. Any place covered by an arch; an archway; as, to pass into
      the arch of a bridge.

   4. Any curvature in the form of an arch; as, the arch of the
      aorta. ``Colors of the showery arch.'' --Milton.

   {Triumphal arch}, a monumental structure resembling an arched
      gateway, with one or more passages, erected to commemorate
      a triumph.

Source : WordNet®

triumphal arch
     n : a monumental archway; usually they are built to commemorate
         some notable victory