synonymy

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}]
    

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]
    

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