In by Jackie Dunn

A device that relegates the temperature of a room or building by switching heating or cooling equipment on or off.

Merriam-Webster Online Dictionary
thermostat (noun)
an automatic device for regulating temperature (as by controlling the supply of gas or electricity to a heating apparatus) , also a similar device for actuating fire alarms or for controlling automatic sprinklers
thermostat (verb)
transitive verb
to provide with or control the temperature of by a thermostat
thermostat (Wikipedia)

Honeywell's iconic "The Round" model T87 thermostat, one of which is in the Smithsonian.
Next Generation Lux Products TX9600TS Universal 7-Day Programmable Touch Screen Thermostat.
A Honeywell electronic thermostat in a retail store

A thermostat is a component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint.

Thermostats are used in any device or system that heats or cools to a setpoint temperature, examples include building heating, central heating, air conditioners, HVAC systems, water heaters, as well as kitchen equipment including ovens and refrigerators and medical and scientific incubators. In scientific literature, these devices are often broadly classified as thermostatically controlled loads (TCLs). Thermostatically controlled loads comprise roughly 50% of the overall electricity demand in the United States.

A thermostat operates as a "closed loop" control device, as it seeks to reduce the error between the desired and measured temperatures. Sometimes a thermostat combines both the sensing and control action elements of a controlled system, such as in an automotive thermostat.

The word thermostat is derived from the Greek words θερμός thermos, "hot" and στατός statos, "standing, stationary".

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