A thermostat is a <a href="https://en.wikipedia.org/wiki/Measuring_instrument" target="_blank" rel="nofollow noopener noreferrer">Measuring Instrument</a> for regulating the <a href="https://en.wikipedia.org/wiki/Temperature" target="_blank" rel="nofollow noopener noreferrer">Temperature</a> of a <a href="https://en.wikipedia.org/wiki/System" target="_blank" rel="nofollow noopener noreferrer">System</a> so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by controlling the <a href="https://en.wikipedia.org/wiki/Flow" target="_blank" rel="nofollow noopener noreferrer">Flow</a> of heat <a href="https://en.wikipedia.org/wiki/Energy" target="_blank" rel="nofollow noopener noreferrer">Energy</a> into or out of the system. That is, the thermostat switches heating or cooling devices on or off as needed to maintain the correct temperature.
A thermostat may be a control unit for a heating or cooling system or a component part of a heater or air conditioner. Thermostats can be constructed in many ways and may use a variety of sensors to measure the temperature. The output of the sensor then controls the heating or cooling apparatus.


well.. try to type into wikipedia ^_^