A coffer (or coffering) in architecture, is a series of sunken panels in the shape of a square, rectangle, or octagon in a ceiling, soffit or vault. A series of these sunken panels were used as decoration for a ceiling or a vault, also called caissons ('boxes“), or lacunaria (“spaces, openings”), so that a coffered ceiling can be called a lacunar ceiling: the strength of the structure is in the framework of the coffers. (source)