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.

• the transposition of letters, syllables or sounds within a word
  
• (chemistry) the double decomposition of inorganic salts
  
• (chemistry) the breaking and reforming of double bonds in olefins in which substituent groups are swapped