SYNONYMY.
1. Equivalence in meaning, as with enormous and immense, both having the general sense ‘very big’. Such equivalence may be precise, as is generally the case with these adjectives, or may be relative, as with big and large: the phrases a big house and a large house may communicate the same idea of size, but in That was a big help and He was in large part to blame for what happened, the two words are not interchangeable.

2. A set, list, system, or book of synonyms. There have been many such works since the 18c, including books that seek through short essays to make discriminations among the members of sets of words. One of the earliest of these in English was Hester Thrale Piozzi's The British Synonymy (1794), which set the style for later works of the same kind. George Crabb brought out in 1816 a detailed work entitled English Synonyms Explained in Alphabetic Order, containing etymologies, citations, and discursive personal comment, and in 1851 Elizabeth Jane Whately published A Selection of English Synonyms, arguing that words in need of discrimination are not synonyms as such but ‘pseudo-synonyms’. Since the mid-19c, alphabetically ordered synonymies are usually called dictionaries, and include Richard Soule's A Dictionary of English Synonyms (1871, revised by A. D. Sheffield, 1937: lists only), Webster's New Dictionary of Synonyms (ed. Rose Egan, 1968: largely with discriminating statements and described on the cover as a ‘thesaurus’), and the Longman Synonym Dictionary (ed. Laurence Urdang, 1979: without discriminations but with usage labels). See THESAURUS.

syn·on·y·my / səˈnänəmē/ • n. the state of being synonymous.

Synonymy

synonyms collectively, 1683.