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}]