A device that keeps something at the temperature you chose. You set the number you want, and the thermostat watches the real temperature and switches the heating or cooling on and off by itself to hold it there. Your air conditioner has one, your fridge has one, your electric kettle and water heater have one, and in cold countries there is one on the wall that controls the whole house's heating. Many car engines have one too, controlling how the cooling water flows. In everyday talk the word usually means that dial or panel you touch: 'turn the thermostat down', 'set the thermostat to 26 degrees'. The name says exactly what it does — it makes heat stand still. English speakers sometimes use it for a person: 'she's the thermostat of the office' means she is the one who keeps everyone's mood steady. That use is informal and not very common.