before vowels therm-, word-forming element meaning "hot, heat, temperature," used in scientific and technical words, from Greek thermos "hot, warm," therme "heat" (from PIE root *gwher- "to heat, warm").
word-forming element used in making names of devices for stabilizing or regulating (such as thermostat), from Greek statos "standing, stationary," from PIE *ste-to-, suffixed form of root *sta- "to stand, make or be firm." First used in heliostat "an instrument for causing the sun to appear stationary" (1742). Related: -static.
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Definitions of thermostat from WordNet
control the temperature with a thermostat;
a regulator for automatically regulating temperature by starting or stopping the supply of heat;