Source : Webster's Revised Unabridged Dictionary (1913)

Synonymous \Syn*on"y*mous\, a. [Gr. ?; sy`n with, together + ?,
   ?, name. See {Syn-}, and {Name}.]
   Having the character of a synonym; expressing the same thing;
   conveying the same, or approximately the same, idea. --
   {Syn*on"y*mous*ly}, adv.

         These words consist of two propositions, which are not
         distinct in sense, but one and the same thing variously
         expressed; for wisdom and understanding are synonymous
         words here.                              --Tillotson.

   Syn: Identical; interchangeable. -- {Synonymous},
        {Identical}. If no words are synonymous except those
        which are identical in use and meaning, so that the one
        can in all cases be substituted for the other, we have
        scarcely ten such words in our language. But the term
        more properly denotes that the words in question
        approach so near to each other, that, in many or most
        cases, they can be used interchangeably. 1. Words may
        thus coincide in certain connections, and so be
        interchanged, when they can not be interchanged in other
        connections; thus we may speak either strength of mind
        or of force of mind, but we say the force (not strength)
        of gravitation. 2. Two words may differ slightly, but
        this difference may be unimportant to the speaker's
        object, so that he may freely interchange them; thus it
        makes but little difference, in most cases, whether we
        speak of a man's having secured his object or having
        attained his object. For these and other causes we have
        numerous words which may, in many cases or connections,
        be used interchangeably, and these are properly called
        synonyms. Synonymous words ``are words which, with great
        and essential resemblances of meaning, have, at the same
        time, small, subordinate, and partial differences, --
        these differences being such as either originally and on
        the ground of their etymology inhered in them; or
        differences which they have by usage acquired in the
        eyes of all; or such as, though nearly latent now, they
        are capable of receiving at the hands of wise and
        discreet masters of the tongue. Synonyms are words of
        like significance in the main, but with a certain
        unlikeness as well.'' --Trench.

Source : WordNet®

synonymous
     adj : (of words) meaning the same or nearly the same [ant: {antonymous}]