Let us return to our simple example of the temperature-controlled room with the thermometer, temperature controller and heater only. We will use this example to highlight some terms commonly used to describe control systems.

In the above example, the room temperature is the variable which is controlled by the system and is therefore referred to as the controlled variable.

The thermometer is the sensor which measures the temperature in the room. The temperature controller takes the information from the sensor and turns the heater on or off depending on the set-point. The heater is called an effector since it is the device which causes changes in room temperature.