The Collaborative International Dictionary of English v.0.48:

Admixture \Ad*mix"ture\ (?; 135), n. [L. admiscere, admixtum, to
   admix; ad + miscere to mix. See Mix.]
   [1913 Webster]
   1. The act of mixing; mixture.
      [1913 Webster]

   2. The compound formed by mixing different substances
      together.
      [1913 Webster]

   3. That which is mixed with anything.
      [1913 Webster]

WordNet (r) 3.0 (2006):

admixture
    n 1: the state of impairing the quality or reducing the value of
         something [syn: admixture, alloy]
    2: an additional ingredient that is added by mixing with the
       base; "the growing medium should be equal parts of sand and
       loam with an admixture of peat moss and cow manure"; "a large
       intermixture of sand" [syn: admixture, intermixture]
    3: the act of mixing together; "paste made by a mix of flour and
       water"; "the mixing of sound channels in the recording
       studio" [syn: mix, commixture, admixture, mixture,
       intermixture, mixing]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "admixture":
   accretion, addition, alloy, alloyage, amalgam, amalgamation, bit,
   blend, blending, coalescence, combination, combo, comminglement,
   commingling, commixture, composite, composition, compound,
   concoction, confection, dash, doctor, eclecticism, ensemble,
   fortification, fusion, immixture, integration, interfusion,
   interlarding, interlardment, interminglement, intermingling,
   intermixture, magma, merger, mingling, mix, mix-up, mixing,
   mixture, paste, pluralism, shade, smack, spice, syncretism, taint,
   tinge