Source : Webster's Revised Unabridged Dictionary (1913)

Tonometer \To*nom"e*ter\, n. [Gr. ? a stretching, a tone +
   -meter.]
   1. (Physics.) An instrument for determining the rate of
      vibrations in tones.

   2. (Physiol.)
      (a) An apparatus for studying and registering the action
          of various fluids and drugs on the excised heart of
          lower animals.
      (b) An instrument for measuring tension, esp. that of the
          eyeball.

Source : WordNet®

tonometer
     n : measuring instrument for measuring tension or pressure
         (especially for measuring intraocular pressure in testing
         for glaucoma)