The Collaborative International Dictionary of English v.0.48:

Decoy \De*coy"\, n.
   1. Anything intended to lead into a snare; a lure that
      deceives and misleads into danger, or into the power of an
      enemy; a bait.
      [1913 Webster]

   2. A fowl, or the likeness of one, used by sportsmen to
      entice other fowl into a net or within shot.
      [1913 Webster]

   3. A place into which wild fowl, esp. ducks, are enticed in
      order to take or shoot them.
      [1913 Webster]

   4. A person employed by officers of justice, or parties
      exposed to injury, to induce a suspected person to commit
      an offense under circumstances that will lead to his
      detection.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Decoy \De*coy"\ (d[-e]*koi"), v. t. [imp. & p. p. Decoyed
   (-koid"); p. pr. & vb. n. Decoying.] [Pref. de- + coy;
   orig., to quiet, soothe, caress, entice. See Coy.]
   To lead into danger by artifice; to lure into a net or snare;
   to entrap; to insnare; to allure; to entice; as, to decoy
   troops into an ambush; to decoy ducks into a net.
   [1913 Webster]

         Did to a lonely cot his steps decoy.     --Thomson.
   [1913 Webster]

         E'en while fashion's brightest arts decoy,
         The heart, distrusting, asks if this be joy.
                                                  --Goldsmith.

   Syn: To entice; tempt; allure; lure. See Allure.
        [1913 Webster]

WordNet (r) 3.0 (2006):

decoy
    n 1: a beguiler who leads someone into danger (usually as part
         of a plot) [syn: decoy, steerer]
    2: something used to lure fish or other animals into danger so
       they can be trapped or killed [syn: bait, decoy, lure]
    v 1: lure or entrap with or as if with a decoy

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "decoy":
   Dionaea, agent provocateur, allure, allurement, attract,
   attraction, bait, bait the hook, baited trap, birdlime, blandish,
   booby trap, by-bidder, cajole, capper, catch, catch out, charm,
   chicane, chicanery, coax, come-on, come-on man, deadfall,
   deathtrap, deceive, deception, decoy duck, delude, draw, draw in,
   draw on, drawcard, drawing, drawing card, endearment, enmesh,
   ensnare, ensnarl, entangle, entice, enticement, entoil, entrap,
   enweb, firetrap, flirt, flirt with, flytrap, gin, give the come-on,
   ground bait, hook, hook in, induce, inducement, inveigle,
   inveiglement, lead on, lime, lure, mesh, mine, mislead, mole trap,
   mousetrap, net, noose, offer bait to, pitfall, plant, rattrap,
   rope in, seduce, seducement, set gun, shill, snare, snarl, sniggle,
   spread the toils, spring gun, stall, stool pigeon, stoolie,
   suck in, tangle, tempt, temptation, toll, trap, trapfall, trick,
   trickery, trip, wile, wind, woo



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DECOY. A pond used for the breeding and maintenance of water-fowl. 11 Mod.
74, 130; S. C. 3 Salk. 9; Holt, 14 11 East, 571.