Complex system
A complex system is a system that exhibits some (and possibly all) of the following characteristics:
feedback loops;
some degree of spontaneous order;
robustness of the order;
emergent organization;
numerosity;
hierarchical organization.
A complex system can be also viewed as a system composed on many components which may interact with each other. In many cases it is useful to represent such a system as a network where the nodes represent the components and the links their interactions.
Examples of complex systems are Earth's global climate, the human brain, social organization, an ecosystem, a living cell, and ultimately the entire universe.
History
Although it is arguable that humans have been studying complex systems for thousands of years, the modern scientific study of complex systems is relatively young in comparison to conventional fields of science with simple system assumptions, such as physics and chemistry. The history of the scientific study of these systems follows several different research trends.