in geometry, any part of the circumference of a circle or curved line, lying from one point to another, by which the quantity of the whole circle or line, or some other thing sought after, may be gathered. See GEOMETRY.

in architecture, a concave building erected for the purposes of supporting some structure, or for making an easy passage over rivers. See ARCHITECTURE.

Triumphal Arch, a stately gate of a semicircular form, adorned with sculpture, inscriptions, &c. erected in honour of those who had deserved a triumph.

Arch, in composition, signifies chief, or of the first class, as archangel, archbishop, &c.

ARCHÆUS, or ARCHEUS. See ARCHEUS.