The Collaborative International Dictionary of English v.0.48:

Decomposition \De*com`po*si"tion\, n. [Pref. de- (in sense 3
   intensive) + composition: cf. F. d['e]composition. Cf.
   Decomposition.]
   1. The act or process of resolving the constituent parts of a
      compound body or substance into its elementary parts;
      separation into constituent part; analysis; the decay or
      dissolution consequent on the removal or alteration of
      some of the ingredients of a compound; disintegration; as,
      the decomposition of wood, rocks, etc.
      [1913 Webster]

   2. The state of being reduced into original elements.
      [1913 Webster]

   3. Repeated composition; a combination of compounds. [Obs.]
      [1913 Webster]

   Decomposition of forces. Same as Resolution of forces,
      under Resolution.

   Decomposition of light, the division of light into the
      prismatic colors.
      [1913 Webster]

WordNet (r) 3.0 (2006):

decomposition
    n 1: the analysis of a vector field [syn: decomposition,
         vector decomposition]
    2: in a decomposed state [syn: decomposition,
       disintegration]
    3: (chemistry) separation of a substance into two or more
       substances that may differ from each other and from the
       original substance [syn: decomposition, decomposition
       reaction, chemical decomposition reaction]
    4: (biology) the process of decay caused by bacterial or fungal
       action [syn: decomposition, rot, rotting,
       putrefaction]
    5: the organic phenomenon of rotting [syn: decay,
       decomposition]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "decomposition":
   ablation, atomization, biodegradability, biodegradation, breakup,
   caries, carrion, corrosion, corruption, crumbling, decay,
   degradability, degradation, dilapidation, disintegration,
   disjunction, disorganization, dissolution, dry rot, erosion,
   foulness, gangrene, incoherence, mildew, mold, mortification,
   necrosis, oxidation, oxidization, putrefaction, putrescence,
   putridity, putridness, rancidity, rancidness, rankness,
   ravages of time, resolution, rot, rottenness, rust, slough,
   sphacelation, sphacelus, spoilage, tooth decay, wear,
   wear and tear