The Collaborative International Dictionary of English v.0.48:

Dormer \Dor"mer\, or Dormer window \Dor"mer win"dow\, n.
   [Literally, the window of a sleeping apartment. F. dormir to
   sleep. See Dormant, a. & n.] (Arch.)
   A window pierced in a roof, and so set as to be vertical
   while the roof slopes away from it. Also, the gablet, or
   houselike structure, in which it is contained.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dormant \Dor"mant\, n. [See Dormant, a.] (Arch.)
   A large beam in the roof of a house upon which portions of
   the other timbers rest or " sleep." --Arch. Pub. Soc. --
   Called also dormant tree, dorman tree, dormond, and
   dormer. --Halliwell. Dormer

WordNet (r) 3.0 (2006):

dormer
    n 1: a gabled extension built out from a sloping roof to
         accommodate a vertical window [syn: dormer, dormer
         window]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "dormer":
   bay, bay window, bow window, casement, casement window, fan window,
   fanlight, grille, lancet window, lantern, lattice, light,
   louver window, oriel, pane, picture window, port, porthole,
   rose window, skylight, transom, wicket, window, window bay,
   window glass, windowpane