The central concept of systems theory and the systems approach as a whole is the notion of a system. Many authors have analyzed this concept and developed definitions of a system, varying in degree of formalization. A definition is a linguistic model of a system, and consequently, differences in the model's purposes and requirements lead to different definitions.
The scientific literature presents many definitions of a system, both general and specific. These definitions reflect different areas of activity — scientific, engineering, social, or economic. Accordingly, three distinct perspectives on the definition of a system can be distinguished:
- the system is regarded as an interconnected complex of material objects — this approach is convenient for studying natural objects or material production processes;
- the system consists of two distinct elements: (1) a set of material objects, and (2) information about the states of these objects. This distinction forms the basis for describing the processes involved in material production management;
- the system is regarded purely as an informational entity, that is, as a certain complex of relations (connections, information). This approach is used in problems related to socioeconomic relations and management processes.
System is a term used when one wishes to characterize the object under study or design as something whole, interconnected, and complex — something that cannot be immediately presented in full and exhaustive form, shown visually, depicted graphically, or described by a mathematical expression, formula, equation, or the like.
At different stages of representing an object as a system, in various specific situations, different definitions of a system may be used. As understanding deepens and one moves to a higher level of description or to a different stage of investigation, the definition of a system not only may but should be refined.
At the highest level of abstraction, at which the properties of all systems are generalized, two mutually complementary definitions of a system can be given, corresponding to the two most important aspects of human activity — the cognition of reality and its reciprocal influence:
- a system is the reflection in the consciousness of a subject of the properties of objects and their relationships in the task of research and cognition;
- a system is a method by which a subject interacts with objects, using their properties and the relationships among them, for purposes such as design, operation, or management.