n. In architecture: A frame for glass, as forming a window or part of a window, and made to open by swinging on hinges which are generally affixed to a vertical side of the opening into which it is fitted.n. A compartment between the mullions of a windown. A deep hollow molding used chiefly in cornices, and similar to the scotia of classical or cavetto of Italian architecture. Oxford Glossary. Sometimes, erroneously, casemate.