The Collaborative International Dictionary of English v.0.48:

Thermometer \Ther*mom"e*ter\ (th[~e]r*m[o^]m"[-e]*t[~e]r), n.
   [Thermo- + -meter: cf. F. thermom[`e]tre. See Thermal.]
   (Physics)
   An instrument for measuring temperature, founded on the
   principle that changes of temperature in bodies are
   accompanied by proportional changes in their volumes or
   dimensions.
   [1913 Webster]

   Note: The thermometer usually consists of a glass tube of
         capillary bore, terminating in a bulb, and containing
         mercury or alcohol, which expanding or contracting
         according to the temperature to which it is exposed,
         indicates the degree of heat or cold by the amount of
         space occupied, as shown by the position of the top of
         the liquid column on a graduated scale. See
         Centigrade, Fahrenheit, and R['e]aumur.
         [1913 Webster] To reduce degrees Fahrenheit to degrees
         Centigrade, subtract 32[deg] and multiply by 5/9; to
         reduce degrees Centigrade to degrees Fahrenheit,
         multiply by 9/5 and add 32[deg].
         [1913 Webster]

   Air thermometer, Balance thermometer, etc. See under
      Air, Balance, etc.

   Metallic thermometer, a form of thermometer indicating
      changes of temperature by the expansion or contraction of
      rods or strips of metal.

   Register thermometer, or Self-registering thermometer, a
      thermometer that registers the maximum and minimum of
      temperature occurring in the interval of time between two
      consecutive settings of the instrument. A common form
      contains a bit of steel wire to be pushed before the
      column and left at the point of maximum temperature, or a
      slide of enamel, which is drawn back by the liquid, and
      left within it at the point of minimum temperature.
      [1913 Webster] Thermometric

WordNet (r) 3.0 (2006):

thermometer
    n 1: measuring instrument for measuring temperature

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "thermometer":
   calorimeter, clinical thermometer, cryometer, electric thermometer,
   galvanothermometer, gas thermometer, glass, meat thermometer,
   mercury, pyrometer, pyrometric cone, resistance pyrometer,
   telethermometer, thermal detector, thermocouple,
   thermoelectrometer, thermometrograph, thermostat