noun

1. a crosspiece in a structure; specif.,
   1. a lintel
   2. a horizontal crossbar across the top or middle of a window or the top of a door
2. ☆ a small window or shutterlike panel directly over a door or window, usually hinged to the transom (sense )
3. any crosspiece; specif.,
   1. the horizontal beam of a gallows or cross
   2. any of the transverse beams attached to the sternpost of a wooden ship
   3. the transverse, aftermost part of a boat with a square stern

Etymology: LME traunsom, prob. altered < L transtrum, crossbeam, lit., that which is across < trans: see trans-