Understanding complex systems: Defining an abstract concept

“Understanding complex systems” has become the overarching theme of a new approach to science. Promoting research that encourages a holistic perspective[1] to “understand complex systems” is a long term investment priority in the strategic plan of the National Science Foundation of the United States for the next five years [2]. In 2005 Springer Verlag started a book series entitled “Understanding Complex Systems” [3] and since 2001 the annual “Understanding Complex Systems” symposium is held at the University of Illinois at Urbana-Champaign [4]. The phrase “understanding complex systems” is often used, but its meaning is rarely defined. The phrase is a sequence of three words, where each word describes an abstract concept: understanding, complex, and system.