The Collaborative International Dictionary of English v.0.48:

hypertonic \hy`per*ton"ic\, n. [Pref. hyper- + tonic.]
   (Physiology, Biochemistry)
   Having a higher osmotic pressure than a comparison solution;
   -- of an aqueous solution. Increasing the concentration of
   dissolved solids increases the osmotic pressure, and thus the
   tonicity of a solution. Opposite of hypotonic and
   contrasting with isotonic. [WordNet sense 1]
   [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)

   2. Specifically: Having an osmotic pressure greater than that
      of blood plasma, which is approximately that of a 0.2
      normal solution of sodium chloride. Cells taken from a
      medium of normal physiological osmolality and placed in a
      hypertonic medium will shrink. [WordNet sense 1]
      [PJC]

   3. in a state of abnormally high tension; -- of muscle.
      [WordNet sense 2]

   Syn: pumped-up.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

hypertonic
    adj 1: (of living tissue) in a state of abnormally high tension;
           "hypertonic muscle tissue" [ant: hypotonic]
    2: (of a solution) having a higher osmotic pressure than a
       comparison solution [ant: hypotonic, isosmotic,
       isotonic]