In control and electrical engineering, a control system is an interconnection between two systems that are referred to as the plant and the controller. The plant is the system that is to be controlled (whose behavior is to be controlled or "shaped") while the controller is the system which does the controlling and implements the control action. The plant and controller are assumed to have one or more ports that can be used to receive signals from an external system or to transmit signals to an external system. Interconnection between the plant and controller is achieved by feeding signals from a set of ports on the controller into another set of ports on the plant and, not in all circumstances, vice versa. This interconnection between the plant and controller serves to couple the dynamics of the two systems and facilitates the control action on the plant by the controller.
In practice, the term system refers to a causal (non-anticipating) dynamical system with causality roughly meaning that no signal in the system that is functionally dependent on a set of other signals in the system can depend on the future values of the latter signals, but only on their past values. Causality is a natural characteristic of physical systems since no such systems can predict the future.