The architectural member surrounding the curved opening of an arch, corresponding to the architrave in the case of a square opening.

the moulding around an arch.

A moulding carried around an arch

One of several parallel curved, and often decorated, mouldings on the inside of an arched opening; a curved architrave.

An archivolt is a group of mouldings (or other elements) surrounding an arched opening, corresponding to the architrave in the case of a rectangular opening. Apart from the decoration, the word sometimes refers to the under-side or inner curve of the arch itself (more properly, the intrados).