in architecture, space in a church above the nave arcade, below the clerestory, and extending over the vaults, or ceilings, of the side aisles. The term is sometimes applied to any second-floor gallery opening onto a higher nave by means of arcades or colonnades, like the galleries in many ancient Roman basilicas or Byzantine churches. The triforium became an integral part of church design during the Romanesque period, serving to light and ventilate the roof space. With the development of the Gothic vaulting system in France, the triforium diminished in size and importance. The cathedrals at Reims (begun 1211) and Amiens (1220–47) both have triforia of little relative height but with rich arcading.

The more horizontal English Gothic style shows an important development of the triforium as a decorative element (Angel Choir, Lincoln Cathedral, completed 1282), but the gallery is relatively much higher than in France, often almost equaling the pier arcades. By the end of the 13th century the triforium was usually replaced by greatly heightened clerestory windows.