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]