is a principal compartment or division in the architectural arrangement of a building, marked either by the buttresses or pilasters on the walls, by the disposition of the main ribs of the vaulting of the interior, by the main arches and pillars, the principals of the roof, or by any other leading features that separate it into corresponding portions. The word is also sometimes used for the space between the mullions of a window, properly called a light; it is occasionally found corrupted into day.