Terminology:

A control system is a system of integrated elements whose function is to manage a procedure variable at a desired value or inside a desired range of values.  The control system monitors a procedure variable or variables, and then causes a few actions to occur to manage the desired system parameter.  In the instance of the central heating unit, the system monitors the temperature of the house using a thermostat. While the temperature of the house drops to a preset value, the furnace turns on, giving a heat source. The temperature of the house increases until a switch in the thermostat causes the furnace to turn off.

Two terms that help describe a control system are input and output. Control system input is the stimulus applied to a control system from an external source to generate a specified response from the control system. Within the case of the central heating unit, the control system input is the temperature of the house as monitored through the thermostat.

Control system output is the actual response obtained from a control system.  In the instance above, the temperature dropping to a preset value on the thermostat causes the furnace to turn on, giving heat to raise the temperature of the house.

In the case of nuclear facilities, the input and output are described through the reason of the control system.  Knowledge of the input and output of the control system enables the elements of the system to be identified. A control system might have more than one input or output.