/ˈsɪn.ə.nɪm/·noun·c. 1425 CE, Middle English 'sinonyme', in logical and rhetorical manuscripts·Established
Origin
From Greek sunōnumos ('having the same name'), compounding syn- (together, from PIE *ḱom) and onuma (name, from PIE *h₁nómn̥) — the root that also gives Latin nomen, English name, and the entire -onym metalinguistic family — synonym names a structural relationship that, under rigorous analysis, cannot fully exist: if meaning is differential, no two signs can occupy identical positions in the system.
Definition
A word or phrase that has the same or nearly the same meaning as another word or phrase in the same language.
The Full Story
Greek / Late LatinClassical Greek, entered English c. 1400swell-attested
The word 'synonym' traces to ancient Greek 'sunōnumos' (συνώνυμος), a compound adjective meaning 'having the same name' or 'of like name', built from two distinct elements: the preposition/prefix 'sun-' (σύν, 'together, with, jointly') and 'onuma/onoma' (ὄνυμα/ὄνομα, 'name'). The neuter substantive form 'sunōnumon' (συνώνυμον) was used by Aristotle in the Categories (c. 350 BCE), where he developed a precise technical distinction between synonymous things (those sharing both a name and the same definition), homonymous things (same name, different definition), and paronymous things (name derived from another). This Aristotelian usage was definitional for later
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The -onym suffix descends from PIE *h₁nómn̥, one of the most stable roots across the entire Indo-European family — the same ancestral word gives Latin nomen (→ noun, nominal, nomenclature), English name, Sanskrit nāman, Greek onoma, Gothic namo, and Armenian anun. From this single root, Greek built an entire toolkit of metalinguistic terms: synonym, antonym, homonym, pseudonym, anonymous, acronym, eponym, patronym, toponym. Every one of these words is essentially a theory
*ḱom, meaning 'together, alongside, with', the same root behind Latin 'cum' (with), 'con-/com-' (prefix), and English 'same' (via Proto-Germanic *samaz from PIE *sem-, a related form). The second element, Greek 'onoma/onuma', descends from PIE *h₁nómn̥ (name), one of the most phonologically stable roots in the Indo-European family, attested
medieval Latin grammar texts. Over time the Aristotelian technical sense narrowed from 'sharing a name and definition' to the modern linguistic sense of 'a word with the same or closely similar meaning as another word in the same language'. The family of -onym words — anonymous, antonym, homonym, pseudonym, eponym, patronym — all share this PIE *h₁nómn̥ root. Key roots: *h₁nómn̥ (Proto-Indo-European: "name; the designation by which a person or thing is known — yields Greek onoma/onuma, Latin nomen, Sanskrit nāman, English name, German Name, Gothic namo"), *ḱom (Proto-Indo-European: "together, alongside, with — yields Greek sun-/syn-, Latin cum/con-/com-, Proto-Germanic *ga- (English y-/ge-), and relates to PIE *sem- (same, one) seen in English 'same'"), sun- (σύν) (Ancient Greek: "prepositional prefix: together, with, jointly — appears in syntax, synagogue, synthesis, synchrony, synergy").