homonym is, in the strict sense, one of a group of words that share the same spelling and the same pronunciation but have different meanings. Thus homonyms are simultaneously homographs (words that share the same spelling, irrespective of their pronunciation) and homophones (words that share the same pronunciation, irrespective of their spelling). The state of being a homonym is called homonymy. Examples of homonyms are the pair stalk (part of a plant) and stalk (follow/harass a person) and the pair left (opposite of right) and left (past tense of leave). A distinction is sometimes made between "true" homonyms, which are unrelated in origin, such as skate (glide on ice) and skate (the fish), and polysemous homonyms, or polysemes, which have a shared origin, such as mouth (of a river) and mouth (of an animal).
In non-technical contexts, the term "homonym" may be used (somewhat confusingly) to refer to words that are either homographs or homophones. In this looser sense, the words row (propel with oars) and row (argument) are considered homonyms, as are the words read (peruse) and reed (waterside plant).
Etymology: The word homonym comes from the Greek ὁμώνυμος (homonumos), meaning "having the same name", which is the conjunction of ὁμός (homos), meaning "common, same" and ὄνομα (onoma) meaning "name". Thus, it refers to two or more distinct concepts sharing the "same name" or signifier. Note: for the h sound, see rough breathing and smooth breathing.
Homographs (literally "same writing") are usually defined as words that share the same spelling, regardless of how they are pronounced.[note 1] If they are pronounced the same then they are also homophones (and homonyms) – for example, bark (the sound of a dog) and bark (the skin of a tree). If they are pronounced differently then they are also heteronyms – for example, bow (the front of a ship) and bow (a type of knot). Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.[note 2] If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing"). Homographic examples include rose (flower) and rose (past tense of rise). Heterographic examples include to, too, two, and there, their, they’re.
Heteronyms (literally "different name") are the subset of homographs (words that share the same spelling) that have different pronunciations (and meanings).[note 3] That is, they are homographs which are not homophones. Such words include desert (to abandon) and desert (arid region); row (to argue or an argument) and row (as in to row a boat or a row of seats - a pair of homophones). Heteronyms are also sometimes called heterophones (literally "different sound").
Polysemes are words with the same spelling and distinct but related meanings. The distinction between polysemy and homonymy is often subtle and subjective, and not all sources consider polysemous words to be homonyms. Words such as mouth, meaning either the orifice on one's face, or the opening of a cave or river, are polysemous and may or may not be considered homonyms.
Capitonyms are words that share the same spelling but have different meanings when capitalized (and may or may not have different pronunciations). Such words include polish (to make shiny) and Polish (from Poland); march (organized, uniformed, steady and rhythmic walking forward) and March (the third month of the year in the Gregorian Calendar). However, both polish or march at the beginning of sentences still need to be capitalized.