The Collaborative International Dictionary of English v.0.48:

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.
      [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]

WordNet (r) 3.0 (2006):

transom window
    n 1: a window above a door that is usually hinged to a
         horizontal crosspiece over the door [syn: transom,
         transom window, fanlight]