A complex system is any organized collection of entities interconnected by a complex network of relationships. They are characterized by being incompletely understood, unpredictable and/or large. In modern physics and biology, and the behavioural and social sciences, problems of organized complexity are commonplace and demand new conceptual tools such as a general theory of organization.
Complex systems are high-order, multiple-loop, nonlinear, feedback structures. They have many unexpected and little understood characteristics, making them very different from the simple systems of which people have an intuitive understanding, including: 1. High order: a system of greater than fourth or fifth order begins to enter the range of complex systems. An adequate representation of a social system, even for limited purposes, can be tenth or hundredth order; 2. Multiple loop: possessing upward of three or four interaction (positive or negative) feedback loops of shifting predominance; 3. Nonlinearity: allowing one feedback loop to dominate the system at one time and then causing a shift in this dominance to another part of the system which may produce such different behaviour that the two may seem unrelated.