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).
The word originates in the Italian (or French) equivalents of the English words arch and vault.
Archivolts and tympanum from Strasbourg Cathedral
Archivolts at Speyer Cathedral
Puerta de Loreto en la Catedral de Orihuela
Entrance into the central church of the Church of St. John the Precursor from the gallery. Yaroslavl, 1911