NOUN (3)

1. a small adhesive disk of paste; used to seal letters;

2. a small thin crisp cake or cookie;

3. thin disk of unleavened bread used in a religious service (especially in the celebration of the Eucharist);

The Collaborative International Dictionary of English v.0.48:

Wafer \Wa"fer\, n. [OE. wafre, OF. waufre, qaufre, F. qaufre; of
   Teutonic origin; cf. LG. & D. wafel, G. waffel, Dan. vaffel,
   Sw. v[*a]ffla; all akin to G. wabe a honeycomb, OHG. waba,
   being named from the resemblance to a honeycomb. G. wabe is
   probably akin to E. weave. See Weave, and cf. Waffle,
   Gauffer.]
   [1913 Webster]
   1. (Cookery) A thin cake made of flour and other ingredients.
      [1913 Webster]

            Wafers piping hot out of the gleed.   --Chaucer.
      [1913 Webster]

            The curious work in pastry, the fine cakes, wafers,
            and marchpanes.                       --Holland.
      [1913 Webster]

            A woman's oaths are wafers -- break with making --B.
                                                  Jonson.
      [1913 Webster]

   2. (Eccl.) A thin cake or piece of bread (commonly
      unleavened, circular, and stamped with a crucifix or with
      the sacred monogram) used in the Eucharist, as in the
      Roman Catholic Church.
      [1913 Webster]

   3. An adhesive disk of dried paste, made of flour, gelatin,
      isinglass, or the like, and coloring matter, -- used in
      sealing letters and other documents.
      [1913 Webster]

   4. Any thin but rigid plate of solid material, esp. of
      discoidal shape; -- a term used commonly to refer to the
      thin slices of silicon used as starting material for the
      manufacture of integrated circuits.
      [PJC]

   Wafer cake, a sweet, thin cake. --Shak.

   Wafer irons, or Wafer tongs (Cookery), a pincher-shaped
      contrivance, having flat plates, or blades, between which
      wafers are baked.

   Wafer woman, a woman who sold wafer cakes; also, one
      employed in amorous intrigues. --Beau. & Fl.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Wafer \Wa"fer\, v. t. [imp. & p. p. Wafered; p. pr. & vb. n.
   Wafering.]
   To seal or close with a wafer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

wafer
    n 1: a small adhesive disk of paste; used to seal letters
    2: a small thin crisp cake or cookie
    3: thin disk of unleavened bread used in a religious service
       (especially in the celebration of the Eucharist)

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "wafer":
   Brussels biscuit, Communion, Eucharist, Holy Communion, Host,
   Last Supper, Melba toast, Sacrament Sunday, altar bread, biscuit,
   bread, bread and wine, coat, coating, collop, consecrated bread,
   consecrated elements, consubstantiation, covering, cracker, cut,
   deal, disk, elements, feuille, film, flap, foil, fold,
   graham cracker, gruel, hardtack, impanation, intinction, lamella,
   lamina, laminated glass, laminated wood, lap, lath, leaf, loaf,
   membrane, mere shadow, pane, panel, paper, patina, peel, pellicle,
   pilot biscuit, plait, plank, plate, plating, ply, plywood, pretzel,
   rail, rake, rasher, real presence, rusk, safety glass, saltine,
   scum, sea biscuit, shadow, shaving, sheet, ship biscuit, sinker,
   skeleton, skin, slab, slat, slice, slip, soda cracker, soup,
   splinter, streak, subpanation, table, tablet, the Holy Sacrament,
   the Sacrament, transubstantiation, vein, veneer, zwieback