complex system

• A complex system is any system featuring a large number of interacting components (agents, processes, etc.) whose aggregate activity is nonlinear (not derivable from the summations of the activity of individual components) and typically exhibits hierarchical self-organization under selective pressures. 
• Instead viewing system apart, it is a system view as a whole. Systems are called complex when simple interactions between parts lead to behavior in which there is the emergence of new order and coherence.
• Complex systems are non-linear and it is impossible to predict the behavior of the whole by analyzing the parts separately. 

http://www.csh.umn.edu/wsh/UnderstandingComplexSystems/index.htm