The Collaborative International Dictionary of English v.0.48:

Bullion \Bul"lion\ (b[.u]l"y[u^]n), n. [Cf. OE. bullyon a hook
   used for fastening the dress, a button, stud, an embossed
   ornament of various kinds, e. g., on the cover of a book, on
   bridles or poitrels, for purses, for breeches and doublets,
   LL. bullio the swelling of boiling water, a mass of gold or
   silver, fr. L. bulla boss, stud, bubble (see Bull an
   edict), or perh. corrupted fr. F. billon base coin, LL.
   billio bullion. Cf. Billon, Billet a stick.]
   1. Uncoined gold or silver in the mass.
      [1913 Webster]

   Note: Properly, the precious metals are called bullion, when
         smelted and not perfectly refined, or when refined, but
         in bars, ingots or in any form uncoined, as in plate.
         The word is often often used to denote gold and silver,
         both coined and uncoined, when reckoned by weight and
         in mass, including especially foreign, or uncurrent,
         coin.
         [1913 Webster]

   2. Base or uncurrent coin. [Obs.]
      [1913 Webster]

            And those which eld's strict doom did disallow,
            And damm for bullion, go for current now.
                                                  --Sylvester.
      [1913 Webster]

   3. Showy metallic ornament, as of gold, silver, or copper, on
      bridles, saddles, etc. [Obs.]
      [1913 Webster]

            The clasps and bullions were worth a thousand pound.
                                                  --Skelton.
      [1913 Webster]

   4. Heavy twisted fringe, made of fine gold or silver wire and
      used for epaulets; also, any heavy twisted fringe whose
      cords are prominent.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bullion
    n 1: a mass of precious metal
    2: gold or silver in bars or ingots

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "bullion":
   bar, button, cast, casting, coin gold, coin silver, copper,
   earth metals, gate, gold, gold dust, gold nugget, ingot, metal,
   metal foil, metal leaf, metalleity, metallicity, metallics,
   metalloid, metalware, metalwork, native metals, nickel,
   noble metals, nugget, pig, precious metals, rare metals, regulus,
   semimetal, sheet metal, silver, sow, yellow stuff



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

BULLION. In its usual acceptation, is uncoined gold or silver, in bars,
plates, or other masses. 1 East, P. C. 188.
     2. In the acts of Congress, the term is also applied to copper properly
manufactured for the purpose of being coined into money. For the acts of
Congress, authorizing the coinage of bullion for private individuals, see
Act of April 2, 1792, s. 14, 1 Story, 230; Act of May 19, 1828, 4 Sharsw.
cont. of Story's Laws U. S. 2120; Act of June 28, 1834, Id. 2376; Act of
January 18, 1837, Id. 2522 to 2529. See, for the English law on the subject
of crimes against bullion, 1 Hawk. P. C. 32 to 41.