The Collaborative International Dictionary of English v.0.48:

Synonymy \Syn*on"y*my\, n. [L. synonymia, Gr. ? a synonym: cf.
   F. synonymie.]
   1. The quality of being synonymous; sameness of meaning.
      [1913 Webster]

   2. A system of synonyms.
      [1913 Webster]

   3. (Rhet.) A figure by which synonymous words are used to
      amplify a discourse.
      [1913 Webster]

WordNet (r) 3.0 (2006):

synonymy
    n 1: the semantic relation that holds between two words that can
         (in a given context) express the same meaning [syn:
         synonymy, synonymity, synonymousness]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "synonymy":
   agreement, biographical dictionary, chemical dictionary,
   coequality, coincidence, congruence, correspondence,
   desk dictionary, dialect dictionary, dictionary,
   dictionary of quotations, electronics dictionary, equality,
   equivalence, etymological dictionary, foreign-language dictionary,
   gazetteer, general dictionary, geological dictionary, gloss,
   glossary, gradus, homogeneity, homoousia, identity,
   indistinguishability, lexicon, no difference, nomenclator, oneness,
   onomasticon, phrase book, polyglot dictionary, rhyming dictionary,
   sameness, science dictionary, self-identity, selfhood, selfness,
   selfsameness, slang dictionary, specialized dictionary,
   synonym dictionary, synonymity, synonymousness, thesaurus,
   treasury of words, unabridged dictionary, unity, vocabulary,
   wordbook