Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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