The Collaborative International Dictionary of English v.0.48:

Thermostat \Ther"mo*stat\, n. [Thermo- + Gr. ? to make to
   stand.] (Physics)
   A device which automatically regulates temperature, or
   provides a signal used by another device to regulate
   temperature. The temperature-sensitive signal may be
   electronic, as that produced by a thermocouple. The signal
   may also be caused mechanically, as by the unequal expansion
   of different metals, liquids, or gases by heat, which can
   then cause the opening or closing of the damper of a stove,
   or the like, as the heat becomes greater or less than is
   desired.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

thermostat
    n 1: a regulator for automatically regulating temperature by
         starting or stopping the supply of heat [syn: thermostat,
         thermoregulator]
    v 1: control the temperature with a thermostat