The Collaborative International Dictionary of English v.0.48:

Metathesis \Me*tath"e*sis\, n.; pl. Metatheses. [L., fr. Gr.
   meta`qesis, fr. metatiqe`nai to place differently, to
   transpose; meta` beyond, over + tiqe`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.
      [1913 Webster]

   2. (Med.) A mere change in place of a morbid substance,
      without removal from the body.
      [1913 Webster]

   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.
      [1913 Webster] Metathetic

WordNet (r) 3.0 (2006):

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, metathesis]