The Collaborative International Dictionary of English v.0.48:

Gable \Ga"ble\, n. [OE. gable, gabil, F. gable, fr. LL. gabalum
   front of a building, prob. of German or Scand. origin; cf.
   OHG. gibil, G. giebel gable, Icel. gafl, Goth. gibla
   pinnacle; perh. akin to Gr. ? head, and E. cephalic, or to G.
   gabel fork, AS. geafl, E. gaffle, L. gabalus a kind of
   gallows.] (Arch.)
   (a) The vertical triangular portion of the end of a building,
       from the level of the cornice or eaves to the ridge of
       the roof. Also, a similar end when not triangular in
       shape, as of a gambrel roof and the like. Hence:
   (b) The end wall of a building, as distinguished from the
       front or rear side.
   (c) A decorative member having the shape of a triangular
       gable, such as that above a Gothic arch in a doorway.
       [1913 Webster]

   Bell gable. See under Bell.

   Gable roof, a double sloping roof which forms a gable at
      each end.

   Gable wall. Same as Gable
   (b) .

   Gable window, a window in a gable.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gable wall
    n 1: the vertical triangular wall between the sloping ends of
         gable roof [syn: gable, gable end, gable wall]