transom

n. [ Probably fr. L. transtrum a crossbeam, transom, from trans across. Cf. Trestle. ] 1. (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. [ 1913 Webster ]

2. (Naut.) One of the principal transverse timbers of the stern, bolted to the sternpost and giving shape to the stern structure; -- called also transsummer. [ 1913 Webster ]

3. (Gun.) The piece of wood or iron connecting the cheeks of some gun carriages. [ 1913 Webster ]

4. (Surg.) The vane of a cross-staff. Chambers. [ 1913 Webster ]

5. (Railroad) One of the crossbeams connecting the side frames of a truck with each other. [ 1913 Webster ]

Transom knees (Shipbuilding), knees bolted to the transoms and after timbers. --
Transom window. (Arch.) (a) A window divided horizontally by a transom or transoms. (b) A window over a door, with a transom between. [ 1913 Webster ]