Complex System
A system composed of many components which may interact with each other. Examples of complex systems are the human brain specifically, and many biological systems generally, transportation or communication systems, social and economic organizations (like cities), and the whole universe.
Origin
American mathematician Warren Weaver introduced the concept in his 1948 essay "Science and Complexity," published while he served as director of natural sciences at the Rockefeller Foundation. Weaver distinguished between "problems of simplicity," "problems of disorganized complexity," and a crucial "great middle region" of "problems of organized complexity"—systems where individual parts act independently yet follow simple rules. His framework laid the groundwork for complexity theory, though the first dedicated research institute, the Santa Fe Institute, wasn't founded until 1984.