metathesis

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Linguistics Transposition within a word of letters, sounds, or syllables, as in the change from Old English brid to modern English bird or in the confusion of modren for modern.
2. n. Chemistry Double decomposition.

Century Dictionary and Cyclopedia

1. n. In grammar, transposition, more especially of the letters, sounds, or syllables of a word, as in the case of Anglo-Saxon ācsian, ās-cian, English ax, ask; Anglo-Saxon brid, English bird.
2. n. In surgery, a change in place of a morbid substance; an operation removing a morbific agent from one part to another, as in couching for cataract
3. n. In logic, same as conversion.

Wiktionary

1. n. prosody the transposition of letters, syllables or sounds within a word, such as in ask as /æks/
2. n. inorganic chemistry the double decomposition of inorganic salts
3. n. organic chemistry the breaking and reforming of double bonds in olefins in which substituent groups are swapped

GNU Webster's 1913

1. n. (Gram.) Transposition, as of the letters or syllables of a word
2. n. (Med.) A mere change in place of a morbid substance, without removal from the body.
3. n. (Chem.) The act, process, or result of exchange, substitution, or replacement of atoms and radicals.

WordNet 3.0

1. n. a chemical reaction between two compounds in which parts of each are interchanged to form two new compounds (AB+CD=AD+CB)
2. n. a linguistic process of transposition of sounds or syllables within a word or words within a sentence