The Collaborative International Dictionary of English v.0.48:

Tonometer \To*nom"e*ter\, n. [Gr. ? a stretching, a tone +
   -meter.]
   1. (Physics.) An instrument for determining the rate of
      vibrations in tones.
      [1913 Webster]

   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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

tonometer
    n 1: measuring instrument for measuring tension or pressure
         (especially for measuring intraocular pressure in testing
         for glaucoma)