homonym

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One of two or more words that have the same sound and often the same spelling but differ in meaning, such as bank (embankment) and bank (place where money is kept).
2. n. A word used to designate several different things.
3. n. A namesake.
4. n. Biology A taxonomic name identical to one previously applied to a different species or genus and therefore unacceptable in its new use.

Century Dictionary and Cyclopedia

1. n. One word used to express distinct meanings, or applied as a name to different things: as, Heteropus is a homonym of eight different genera.
2. n. In philology, a word which agrees with another in sound, and perhaps in spelling, but is not the same in meaning; a homophone: as, meet, meat, and mete, or the verb bear and the noun bear. The term is also loosely extended to include words spelled alike but pronounced differently, as bow, bend, bow, a weapon; lead, conduct, lead, a metal, etc. The words so designated may be akin or even ultimately identical in origin, as air, air, bow, bow, meet, meet. See homophone, 2, homograph, 1.
3. n. Specifically, in systematic biology, a name given to a group (usually a genus or species) at a later date than that at which the same name had been given to another group. Such a name is said to be preoccupied. In order to avoid confusion with the earlier names, all homonyms are rejected. Thus the use of Torreya by Rafinesque in 1818 as the name of a genus of plants belonging to the family Menthaceæ prevents the recognition of Torreya, published by Arnott in 1838, as a valid name for a genus of the family Taxaceæ, the latter genus consequently taking its next older name Tumion, published in 1840. Similarly Agriotherium was used by Wagner for a genus of carnivores and by Scott for a genus of ungulates; and Brachyurus was applied by Fischer to a genus of rodents and by Spix to a genus of monkeys.

Wiktionary

1. n. semantics (strict sense) A word that both sounds and is spelled the same as another word but has a different meaning.
2. n. loosely A word that sounds or is spelled the same as another word but has a different meaning, technically called a homophone (same sound) or a homograph (same spelling).
3. n. taxonomy A name for a taxon that is identical in spelling to another name that belongs to a different taxon.

GNU Webster's 1913

1. n. A word having the same sound as another, but differing from it in meaning; as the noun bear and the verb bear.

WordNet 3.0

1. n. two words are homonyms if they are pronounced or spelled the same way but have different meanings