Some definitions
We can find in the literature different definitions of the complex system :
Joel Moses [Aiguier et al, 2010][1] : suggests that the complexity is composed of many parts that interconnect in intricate ways. The complexity of the system is related to the number of interactions and to their nature.
Peter Senge [Senge, P. M, 2014][2] : Dynamic Complexity occurs when obvious interventions produce non-obvious consequences.
Joseph Sussman [Sussman, J. M, 1999][3] : suggests that a system is complex when it is composed of a group of related units for which the degree and nature of the relationships is imperfectly known.
The previous definitions can be completed by adding that in complex system not only composed of wide range of scales but also the different structure interact (non-linearly) each other.