From 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
From 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}]