Source : Webster's Revised Unabridged Dictionary (1913)

Transom \Tran"som\, 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}.

   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}.

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

   4. (Surg.) The vane of a cross-staff. --Chambers.

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

   {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.

Source : WordNet®

transom
     n 1: a window above a door and hinged to a transom [syn: {transom
          window}, {fanlight}]
     2: a horizontal crosspiece across a window or separating a door
        from a window over it [syn: {traverse}]