1. [n]  (architecture) a masonry construction (usually curved) for spanning an opening and supporting the weight above it
2. [n]  a passageway under an arch
3. [n]  a curved bony structure supporting or enclosing organs (especially arches of the feet)
4. [n]  a curved shape in the vertical plane that spans an opening
5. [adj]  (used of behavior or attitude) characteristic of those who treat others with condescension
6. [v]  form an arch

Arch
Renowned outdoor materials artist Andy Goldsworthy offers an inside look at an intriguing project: the building, dismantling, and rebuilding of a red sandstone arch in a variety of locations between Scotland and England. 30 color illustrations.

more details ...

1. \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.
   

2. \Arch\, v. t. [imp. & p. p. {Arched}; p. pr. & vb. n.
   {Arching}.]
   1. To cover with an arch or arches.
   2. To form or bend into the shape of an arch.
            The horse arched his neck.            --Charlesworth.
   

3. \Arch\, v. i.
   To form into an arch; to curve.
   

4. \Arch-\ (["a]rch-, except in archangel and one or two
   other words). [L. arch-, Gr. ?. See {Arch-}.]
   A prefix signifying chief, as in archbuilder, archfiend.
   

5. \Arch\ (["a]rch), a. [See {Arch-}, pref.]
   1. Chief; eminent; greatest; principal.
            The most arch act of piteous massacre. --Shak.
   2. Cunning or sly; sportively mischievous; roguish; as, an
      arch look, word, lad.
            [He] spoke his request with so arch a leer.
                                                  --Tatler.
   

6. \Arch\, n. [See {Arch-}, pref.]
   A chief. [Obs.]
         My worthy arch and patron comes to-night. --Shak.
   

7. \-arch\ [Gr. 'archo`s chief, commander, 'a`rchein to rule.
   See {Arch}, a.]
   A suffix meaning a ruler, as in monarch (a sole ruler).
   

an architectural term found only in Ezek. 40:16, 21, 22, 26, 29. There is no absolute proof that the Israelites employed arches in their buildings. The arch was employed in the building of the pyramids of Egypt. The oldest existing arch is at Thebes, and bears the date B.C. 1350. There are also still found the remains of an arch, known as Robinson's Arch, of the bridge connecting Zion and Moriah. (See TYROPOEON VALLEY.)