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]

WordNet (r) 3.0 (2006):

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