metathesis

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.
• noun Chemistry Double displacement.

from The Century Dictionary.

• noun 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.
• noun 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
• noun In logic, same as conversion.

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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