A system can be either closed or open:
A closed system is a system that is completely isolated from its environment.
This is the definition commonly used in the system literature, which we have chosen to follow. This is different from the thermodynamics definition, which differentiates between systems that are “closed” (no material flow) and “isolated” (no material or energy flow).
The physical universe, as we currently understand it, appears to be a closed system.
An open system is a system that has flows of information, energy, and/or matter between the system and its environment, and which adapts to the exchange.
This a fundamental systems science definition. It differs from the meaning of “open system” in IT and related fields, where the term is used in the sense of “open system architecture” that allows for a vendor-independent, non-proprietary, computer system or device design based on official and/or popular standards.
All physical systems of interest to systems engineering are open systems. However, there can be special cases in systems engineering where it is convenient to treat a system as if it is closed, if there are no significant external relationships or interactions to contend with.
Entropy increases in a closed system. In open systems, the entropy is kept low, or decreases, essentially at the expense of entropy increasing somewhere else, so the entropy of the universe continues to increase. Thus, systems tend to maintain their organisation at the expense of increased disorder elsewhere, which is a common cause of unintended consequences.
It follows that a more fundamental definition of “system” could be “a persistent region of low entropy (= high organisation) in physical or conceptual space-time”. Then, it would follow that “systemness is the phenomenon that allows regions of organisation to persist in a dissipative universe”.