Source : Webster's Revised Unabridged Dictionary (1913)

Pyrometer \Py*rom"e*ter\, n. [Pyro- + -meter: cf. F.
   pyrom[`e]tre.]
   1. (Physics) An instrument used for measuring the expansion
      of solid bodies by heat.

   2. (Physics) An instrument for measuring degrees of heat
      above those indicated by the mercurial thermometer.

   Note: It is usually constructed so as to register the change
         which the heat to be measured produces in the length of
         some expansible substance, as a metallic rod, or in the
         intensity of a thermo-electric current.

Source : WordNet®

pyrometer
     n : a thermometer designed to measure high temperatures