transom

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A horizontal crosspiece over a door or between a door and a window above it.
• noun A small hinged window above a door or another window.
• noun A horizontal dividing bar of wood or stone in a window.
• noun A lintel.
• noun Any of several transverse beams affixed to the sternpost of a wooden ship and forming part of the stern.
• noun The aftermost transverse structural member in a steel ship, including the floor, frame, and beam assembly at the sternpost.
• noun The stern of a square-sterned boat when it is a structural member.
• noun The horizontal beam on a cross or gallows.
• idiom (over the transom) Without being agreed to; unsolicited.

from The Century Dictionary.

• noun In architecture, a horizontal bar of timber or stone across a window; also, the cross-bar separating a door from the fanlight above it. See mullion.
• noun Same as transom-window, 2.
• noun A slat of a bedstead.
• noun Nautical, one of several beams or timbers fixed across the stern-post of a ship to strengthen the after part and give it the figure most suitable to the service for which the vessel is intended. See also cut under counter.
• noun In a saw-pit, a joist resting transversely upon the strakes.
• noun One of two beams of wood or metal secured horizontally to the side frames of a railway car-truck. They are placed one on each side of the swing-bolster.
• noun In gunnery, a piece of wood or iron joining the cheeks of gun-carriages, whence the terms transom-plates, transom-bolts, etc.
• noun In surveying, a piece of wood made to slide upon a cross-staff; the vane of a cross-staff.
• noun One of the cross-ties or sleepers laid under the longitudinal sills of a permanent way for a street railway, or for any railway laid on this system, as in a mine.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Arch.) A horizontal crossbar in a window, over a door, or between a door and a window above it. Transom is the horizontal, as mullion is the vertical, bar across an opening. See Illust. of mullion.
• noun (Naut.) One of the principal transverse timbers of the stern, bolted to the sternpost and giving shape to the stern structure; -- called also transsummer.
• noun (Gun.) The piece of wood or iron connecting the cheeks of some gun carriages.
• noun (Surg.) The vane of a cross-staff.
• noun (Railroad) One of the crossbeams connecting the side frames of a truck with each other.
• noun (Shipbuilding) knees bolted to the transoms and after timbers.
• noun (Arch.) A window over a door, with a transom between.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A crosspiece over a door; a lintel.
• noun A horizontal dividing bar in a window.
• noun nautical Any of several transverse structural members in a ship, especially at the stern; a thwart.
• noun nautical The flat or nearly flat stern of a boat or ship.
• noun The horizontal beam on a cross or gallows.
• noun figuratively Items that have arrived over the transom.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a window above a door that is usually hinged to a horizontal crosspiece over the door
• noun a horizontal crosspiece across a window or separating a door from a window over it