transom

In architecture, a transom is a transverse horizontal structural beam or bar, or a crosspiece separating a door from a window above it. This contrasts with a mullion, a vertical structural member. Transom or transom window is also the customary U.S. word used for a transom light, the window over this crosspiece. In Britain, the ‘transom light’ above a door is usually referred to as a “fanlight” if a semi-elliptical shape, and occasionally as an “overlight” or “hopper”, or by the French word “vasistas”.