The suffix -onym (from Ancient Greek: ὄνυμα, lit. 'name') is a bound morpheme, that is attached to the end of a root word, thus forming a new compound word that designates a particular class of names. In linguistic terminology, compound words that are formed with suffix -onym are most commonly used as designations for various onomastic classes. Most onomastic terms that are formed with suffix -onym are classical compounds, whose word roots are taken from classical languages (Greek and Latin).

For example, onomastic terms like toponym and linguonym are typical classical (or neoclassical) compounds, formed from suffix -onym and classical (Greek and Latin) root words (Ancient Greek: τόπος / place; Latin: lingua / language). In some compounds, the -onym morpheme has been modified by replacing (or dropping) the "o". In the compounds like ananym and metanym, the correct forms (anonym and metonym) were pre-occupied by other meanings. Other, late 20th century examples, such as hypernym and characternym, are typically redundant neologisms, for which there are more traditional words formed with the full -onym (hyperonym and charactonym).

The English suffix -onym is from the Ancient Greek suffix -ώνυμον (ōnymon), neuter of the suffix ώνυμος (ōnymos), having a specified kind of name, from the Greek ὄνομα (ónoma), Aeolic Greek ὄνυμα (ónyma), "name". The form -ōnymos is that taken by ónoma when it is the end component of a bahuvrihi compound, but in English its use is extended to tatpuruṣa compounds.

The suffix is found in many modern languages with various spellings. Examples are: Dutch synoniem, German Synonym, Portuguese sinónimo, Russian синоним (sinonim), Polish synonim, Finnish synonyymi, Indonesian sinonim, Czech synonymum.

According to a 1988 study of words ending in -onym, there are four discernible classes of -onym words: (1) historic, classic, or, for want of better terms, naturally occurring or common words; (2) scientific terminology, occurring in particular in linguistics, onomastics, etc.; (3) language games; and (4) nonce words. Older terms are known to gain new, sometimes contradictory, meanings (e.g., eponym and cryptonym). In many cases, two or more words describe the same phenomenon, but no precedence is discernible (e.g., necronym and penthonym). New words are sometimes created, the meaning of which duplicating existing terms. On occasion, new words are formed with little regard to historical principles.