System (from Latin systēma, in turn from Greek σύστημα systēma) is a set of entities, real or abstract, comprising a whole where each component interacts with or is related to at least one other component and they all serve a common objective. Any object which has no relation with any other element of the system is not part of that system but rather of the system environment. A subsystem then is a set of elements, which is a system itself, and a part of the whole system.
Every division or aggregation of real entities into systems is arbitrary, therefore it is a subjective abstract concept.
The scientific research field which is engaged in the transdisciplinary study of universal system-based properties of the world is general system theory, systems science and recently systemics. They investigate the abstract properties of the matter and mind, their organization, searching concepts and principles which are independent of the specific domain, independent of their substance, type, or spatial or temporal scales of existence.