What is a complex system, after all?

The study of complex systems, although an interdisciplinary endeavor, it is considered as an integrating part of physical sciences. Contrary to the historical fact that the eld is already mature, it still lacks a clear and unambiguous denition of its main object of study. Here, I propose a denition of complex systems based on the conceptual clarications made by Edgar Morin about the bidirectional non-separability of parts and whole produced by the nature of interactions. The concept to which I arrived here is clear, not circular, neither too cryptic nor to explicit, and does not include any accompanying features of the own system dened. It allows, as shown in the paper, to derive some of the main properties that such systems must have as well as to propose its mathematical formalization.