A closed system is a system which relies on a feedback loop to control its output in a control system in systems theory. (It is sometimes called “closed-loop system.”) The system is a “closed” system because it is closed by a feedback loop in its control, as opposed to an open system where there is no feedback loop in its system. In a closed system, part of the output of this system is also used as the input to the system, which is essentially its “feedback.”

A closed system can rely on a positive feedback or a negative feedback or both in its control operation. In contrast, an open system is a feedforward system that does not rely on feedback for its control function.