1. 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.
2. One of the principal transverse timbers of the stern, bolted to the sternpost and giving shape to the stern structure; called also transsummer.
3. The piece of wood or iron connecting the cheeks of some gun carriages.
4. (Science: surgery) The vane of a cross–staff.
5. One of the crossbeams connecting the side frames of a truck with each other. Transom knees A window divided horizontally by a transom or transoms. A window over a door, with a transom between.
Origin: Probably fr. L. Transtrum a crossbeam, transom, from trans across. Cf. Trestle.