Source : Webster's Revised Unabridged Dictionary (1913)

Isotonic \I`so*ton"ic\, a. [Iso- + tonic.]
   Having or indicating, equal tones, or tension.

   {Isotonic system} (Mus.), a system consisting of intervals,
      in which each concord is alike tempered, and in which
      there are twelve equal semitones.

Source : WordNet®

isotonic
     adj 1: (used of solutions) having the same or equal osmotic
            pressure [syn: {isosmotic}] [ant: {hypertonic}, {hypotonic}]
     2: relating to or characterized by the equal intervals of the
        well-tempered scale; "isotonic tuning"
     3: of or involving muscular contraction in which tension is
        constant while length changes [ant: {isometric}]
     4: of two or more muscles; having equal tension