A complex system is one where smaller elements make complex things happen. These smaller elements can be simple elements or they can be complex systems within themselves. Any such complex elements can be interrelated in simple or complex ways and may be hard to identify as distinct.
'Complex systems' is a field which studies the common properties of systems considered complex in nature, society and science. It is also called complex systems theory, complexity science, study of complex systems and/or sciences of complexity.
The key problems of such systems are difficulties with their formal modeling and simulation. From such perspective, in different research contexts complex systems are defined on the base of their different attributes.