An arch is a structure with two main supports which meet above an opening in a span (which is typically but not always curved). The shape of the opening can be a simple arc or an elaborate arc with any number of ornamental indentations; or it can be pointed or rectangular. It should not be confused with arches which form parts of buildings or other structures, such as arched windows or doorways.

This structural form is distinguished from a triumphal arch, which is a specific form of arch inscribed in a rectangular structure.

The opening can be curved, pointed, rectangular, or ornamentally shaped. Its shape does not have to be a simple semicircular arc, but the two supports and connecting span remain the basic arrangement.