System (from the Latin (systēma), and this from the Greek σύστημα (sustēma)) is an assemblage of element comprising a whole with each element related to other elements. Any element which has no relationship with any other element of the system, cannot be a part of that system. A subsystem is then a set of elements which is a proper subset of the whole system.

Every division of an object/entity into systems is arbitrary, therefore it is a subjective abstract concept. The scientific research field which is engaged in the transdisciplinary study of universal systemic properties of the World is the General System Theory or Systems science, it investigates the abstract properties of the matter and mind, their organization, searching concepts and principles which are independent on the specific domain, independent of their substance, type, or spatial or temporal scales of existence.