Triforium In church architecture, an arcaded gallery above the arches of the nave. In the interiors of medieval churches each bay of the nave wall customarily had three divisions in its height: Arcade Triforium Clerestory The triforium was thus located beneath the clerestory windows and above the side-aisle vaults and corresponded on the exterior to the lean-to roof over the aisle. In Italian basilical churches this interior surface was generally decorated with paintings or mosaics. In the north the triforium had arched openings with apertures in the wall behind it to ventilate the roof space over the aisle. In most Romanesque churches it appeared as a second-story vaulted gallery over the aisle and was equal to it in depth and sometimes also in height. In Gothic churches, the depth behind the triforium arcades was generally limited to the thickness of the nave wall, into which a narrow passageway was built to furnish a second-story circulation around the church. Developed French Gothic flattened the pitch of the aisle roofs, thus leaving the outside wall of the triforia exposed and free for glazing. The inside face retained its rich open tracery arcades. Late Gothic subordinated the triforium between the higher main arcades and clerestory and sometimes omitted it entirely. - The Free Dictionary by Farlex

A triforium is an interior gallery, opening onto the tall central space of a building at an upper level. In a church, it opens onto the nave from above the side aisles; it may occur at the level of the clerestory windows, or it may be located as a separate level below the clerestory. Masonry triforia are generally vaulted and separated from the central space by arcades. Early triforia were often wide and spacious, but later ones tend to be shallow, within the thickness of an inner wall, and may be blind arcades not wide enough to walk along. The outer wall of the triforium may itself have windows (glazed or unglazed openings), or it may be solid stone. -  Wikipedia (online Dec. 2019)