Roget's list?

• The semantic relation that holds between two words that can (in a given context) express the same meaning

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Synonymy \Syn*on"y*my\, n. [L. synonymia, Gr. ? a synonym: cf. F. synonymie.] The quality of being synonymous; sameness of meaning. A system of synonyms. (Rhet.) A figure by which synonymous words are used to amplify a discourse.

WordNet Word definitions in WordNet
n. the semantic relation that holds between two words that can (in a given context) express the same meaning [syn: synonymity , synonymousness ]

Wiktionary Word definitions in Wiktionary
n. 1 (context semantics English) The quality of being synonymous; sameness of meaning. 2 A list or collection of synonyms, often compared and contrasted. 3 The study of synonyms. 4 A system of synonyms. 5 (context botany English) The collective synonyms ...

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
1650s, "use of synonyms;" 1794, "quality of being synonymous," from French synonymie and directly from Late Latin synonymia , from Greek synonymos (see synonymous ).