The Collaborative International Dictionary of English v.0.48:

Synonym \Syn"o*nym\ (s[i^]n"[-o]*n[i^]m), n.; pl. Synonyms
   (s[i^]n"[-o]*n[i^]mz). [F. synonyme, L. synonyma, pl. of
   synonymum, Gr. synw`nymon. See Synonymous.]
   1. One of two or more words (commonly words of the same
      language) which are equivalents of each other; one of two
      or more words which have very nearly the same
      signification, and therefore may often be used
      interchangeably. See under Synonymous. [Written also
      synonyme.]
      [1913 Webster]

            All languages tend to clear themselves of synonyms
            as intellectual culture advances, the superfluous
            words being taken up and appropriated by new shades
            and combinations of thought evolved in the progress
            of society.                           --De Quincey.
      [1913 Webster]

            His name has thus become, throughout all civilized
            countries, a synonym for probity and philanthropy.
                                                  --Macaulay.
      [1913 Webster]

            In popular literary acceptation, and as employed in
            special dictionaries of such words, synonyms are
            words sufficiently alike in general signification to
            be liable to be confounded, but yet so different in
            special definition as to require to be
            distinguished. --G. P. Marsh.
      [1913 Webster]

   2. An incorrect or incorrectly applied scientific name, as a
      new name applied to a species or genus already properly
      named, or a specific name preoccupied by that of another
      species of the same genus; -- so used in the system of
      nomenclature (which see) in which the correct scientific
      names of certain natural groups (usually genera, species,
      and subspecies) are regarded as determined by priority.
      [Webster 1913 Suppl.]

   3. One of two or more words corresponding in meaning but of
      different languages; a heteronym. [Rare]
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

synonym
    n 1: two words that can be interchanged in a context are said to
         be synonymous relative to that context [syn: synonym,
         equivalent word] [ant: antonym, opposite, opposite
         word]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "synonym":
   Doppelganger, actual thing, antonym, articulation, carbon copy,
   copy, dead ringer, ditto, double, duplicate, equivalent,
   exact counterpart, expression, facsimile, free form, homograph,
   homonym, homophone, idem, identical same, lexeme, linguistic form,
   locution, logos, metonym, minimum free form, monosyllable,
   no other, none other, polysyllable, replica, selfsame,
   spit and image, spitting image, syllable, term, the same,
   the same difference, twin, usage, utterance, verbalism, verbum,
   very image, very same, vocable, word