The Collaborative International Dictionary of English v.0.48:

mannequin \mannequin\ n.
   1. a woman who wears clothes to display fashions; a fashion
      model. [WordNet sense 1]

   Syn: fashion model, model.
        [WordNet 1.5]

   2. A three-dimensional model of a person, used to display
      clothes, especially in retail stores and in window
      displays; a dummy. [WordNet sense 2]

   Syn: manikin, mannikin, manakin.
        [WordNet 1.5]

   3. A model of a human form used by tailors to help make
      clothing of the proper shape.
      [PJC]

WordNet (r) 3.0 (2006):

mannequin
    n 1: a woman who wears clothes to display fashions; "she was too
         fat to be a mannequin" [syn: mannequin, manikin,
         mannikin, manakin, fashion model, model]
    2: a life-size dummy used to display clothes [syn: mannequin,
       manikin, mannikin, manakin, form]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "mannequin":
   bust, carving, clay model, doll, dolly, dummy, fantoccini, figure,
   figurehead, figurine, gingerbread man, lay figure, man of straw,
   manikin, marionette, mock-up, model, monument, pilot model,
   portrait bust, puppet, scarecrow, sculpture, snowman, statuary,
   statue, statuette, wax figure, waxwork, wood carving, wood model