“-Onym: [Through Latin from Greek onuma/onoma name]. A word base or combining form that stands either for a word (as in synonym) or a name (as in pseudonym). Words containing -onym have two kinds of adjective: with –ous as in synonymous (having the nature or quality of a synonym: synonymous words) or with –ic, as in synonymic (concerning synonyms: synonymic relationships). The form –onymy indicates type, as with synonymy (the type sense relation in which words have the same or similar meaning) and eponymy (the category of word-formation that concerns words derived with people’s names). Because –onym begins with o (the commonest Greek thematic vowel, as in biography), the base form is sometimes taken to be -nym, an assumption reinforced by the initial n of the equivalent terms nomen in Latin and name in English. As a result, some recent technical terms have been formed on –nym: for example, characternym and paranym. See acronym, antonym, eponym, heteronym, homonym, hyponym, retronym.”
Excerpted from: McArthur, Tom. The Oxford Concise Companion to the English Language. New York: Oxford University Press, 2005.