\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.