The Collaborative International Dictionary of English v.0.48:

Isotonic \I`so*ton"ic\, a. [Iso- + tonic.]
   1. Having or indicating, equal tones, or tension.
      [1913 Webster]

   2. (Physiology, Biochemistry) Having an osmotic pressure
      (tonicity) equal to that of a reference solution; -- of
      aqueous solutions. Contrasting with hypotonic and
      hypertonic. See hypertonic.
      [PJC]

            A knowledge of the colligative properties of
            solutions . . . is essential for one to understand
            fully the principles involved in rendering
            intravenous solutions isotonic with blood serum, or
            opthalmic solutions isotonic with lachrymal fluid.
            Solutions thus adjusted produce less shock and much
            less irritation than those which are hypotonic or
            hypertonic, and present-day practise recognizes the
            desirability of making the necessary adjustments
            whenever possible.                    --Cook &
                                                  Martin
                                                  (Remington's
                                                  Practice of
                                                  Pharmacy,
                                                  Tenth Ed.:
                                                  Mack Publ.,
                                                  Easton Pa.,
                                                  1951)

   Isotonic system (Mus.), a system consisting of intervals,
      in which each concord is alike tempered, and in which
      there are twelve equal semitones.
      [1913 Webster]