Transom
Part of speech: noun
Definition: A crosspiece over a door; a lintel. A small hinged window above a door or another window. A horizontal dividing bar in a window. Any of several transverse structural members in a ship, especially at the stern; a thwart. The flat or nearly flat stern of a boat or ship. The horizontal beam on a cross or gallows.
Mullion
Part of speech: noun
Definition: A vertical bar between the panes of glass or casements of a window or the panels of a screen.