Source : Webster's Revised Unabridged Dictionary (1913)

Synonym \Syn"o*nym\, n.
   1. 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.

   2. One of two or more words corresponding in meaning but of
      different languages; a heteronym. [Rare]

Synonym \Syn"o*nym\ (s[i^]n"[-o]*n[i^]m), n.; pl. {Synonyms}
   (-n[i^]mz). [F. synonyme, L. synonyma, pl. of synonymum, Gr.
   synw`nymon. See {Synonymous}.]
   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}.]

         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.

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

         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.

Source : WordNet®

synonym
     n : two words that can be interchanged in a context are said to
         be synonymous relative to that context [syn: {equivalent
         word}]