Source : Webster's Revised Unabridged Dictionary (1913)

Metathesis \Me*tath"e*sis\, n.; pl. {Metatheses}. [L., fr. Gr.
   meta`thesis, fr. metatithe`nai to place differently, to
   transpose; meta` beyond, over + tithe`nai to place, set. See
   {Thesis}.]
   1. (Gram.) Transposition, as of the letters or syllables of a
      word; as, pistris for pristis; meagre for meager.

   2. (Med.) A mere change in place of a morbid substance,
      without removal from the body.

   3. (Chem.) The act, process, or result of exchange,
      substitution, or replacement of atoms and radicals; thus,
      by metathesis an acid gives up all or part of its
      hydrogen, takes on an equivalent amount of a metal or
      base, and forms a salt.

Source : WordNet®

metathesis
     n 1: a linguistic process of transposition of sounds or syllables
          within a word or words within a sentence
     2: a chemical reaction between two compounds in which parts of
        each are interchanged to form two new compounds
        (AB+CD=AD+CB) [syn: {double decomposition}, {double
        decomposition reaction}]
     [also: {metatheses} (pl)]