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
    n 1: a window above a door that is usually hinged to a
         horizontal crosspiece over the door [syn: transom,
         transom window, fanlight]
    2: a horizontal crosspiece across a window or separating a door
       from a window over it [syn: transom, traverse]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "transom":
   air duct, air hole, air passage, air shaft, air tube, airway, bay,
   bay window, blowhole, bow window, breathing hole, casement,
   casement window, cross bitt, crossarm, crossbar, crosspiece,
   doubletree, fan window, fanlight, grille, lancet window, lantern,
   lattice, light, louver, louver window, louverwork, naris, nostril,
   oriel, pane, picture window, port, porthole, rose window, shaft,
   singletree, skylight, spilehole, spiracle, swingletree, touchhole,
   transept, transversal, transverse, traverse, vent, ventage,
   venthole, ventiduct, ventilating shaft, ventilator, whippletree,
   wicket, wind tunnel, window, window bay, window glass,
   windowpane