Source : Webster's Revised Unabridged Dictionary (1913)

Thermostat \Ther"mo*stat\, n. [Thermo- + Gr. ? to make to
   stand.] (Physics)
   A self-acting apparatus for regulating temperature by the
   unequal expansion of different metals, liquids, or gases by
   heat, as in opening or closing the damper of a stove, or the
   like, as the heat becomes greater or less than is desired.

Source : WordNet®

thermostat
     n : a regulator for automatically regulating temperature by
         starting or stopping the supply of heat [syn: {thermoregulator}]
     v : control the temperature with a thermostat