1. a monumental archway; usually they are built to commemorate some notable victory;
The Collaborative International Dictionary of English v.0.48:
Arch \Arch\ ([aum]rch), n. [F. arche, fr. LL. arca, for arcus.
1. (Geom.) Any part of a curved line.
(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
WordNet (r) 3.0 (2006):
n 1: a monumental archway; usually they are built to commemorate
some notable victory