An ornamental molding or band following the curve on the underside of an arch
Look up archivolt in Wiktionary, the free dictionary.
An archivolt (or voussure) is an ornamental molding or band following the curve on the underside of an arch.
It is composed of bands of ornamental moldings (or other architectural elements) surrounding an arched opening, corresponding to the architrave in the case of a rectangular opening. The word is sometimes used to refer to the under-side or inner curve of the arch itself (more properly, the intrados).