A system is a set of interacting or interdependent components forming an integrated whole or a set of elements (often called 'components' ) and relationships which are different from relationships of the set or its elements to other elements or sets.
Fields that study the general properties of systems include Systems science, systems theory, cybernetics, dynamical systems, thermodynamics, and complex systems. They investigate the abstract properties of systems' matter and organization, looking for concepts and principles that are independent of domain, substance, type, or temporal scale.
Some systems share common characteristics, including:
- A system has structure, it contains parts (or components) that are directly or indirectly related to each other;
- A system has behavior, it contains processes that transform inputs into outputs (material, energy or data);
- A system has interconnectivity: the parts and processes are connected by structural and/or behavioral relationships.
- A system's structure and behavior may be decomposed via subsystems and sub-processes to elementary parts and process steps.
The term system may also refer to a set of rules that governs structure and/or behavior. Alternatively, and usually in the context of complex social systems, the term institution is used to describe the set of rules that govern structure and/or behavior.
The word system in its meaning here, has a long history which can be traced back to Plato (Philebus), Aristotle (Politics) and Euclid (Elements). It had meant "total", "crowd" or "union" in even more ancient times, as it derives from the verb sunìstemi, uniting, putting together.
"System" means "something to look at". You must have a very high visual gradient to have systematization. In philosophy, before Descartes, there was no "system". Plato had no "system". Aristotle had no "system". (Marshall McLuhan in: McLuhan: Hot & Cool. Ed. by Gerald Emanuel Stearn. A Signet Book published by The New American Library, New York, 1967, p. 288).
In the 19th century the first to develop the concept of a "system" in the natural sciences was the French physicist Nicolas Léonard Sadi Carnot who studied thermodynamics. In 1824 he studied the system which he called the working substance, i.e. typically a body of water vapor, in steam engines, in regards to the system's ability to do work when heat is applied to it. The working substance could be put in contact with either a boiler, a cold reservoir (a stream of cold water), or a piston (to which the working body could do work by pushing on it). In 1850, the German physicist Rudolf Clausius generalized this picture to include the concept of the surroundings and began to use the term "working body" when referring to the system.
One of the pioneers of the general systems theory was the biologist Ludwig von Bertalanffy. In 1945 he introduced models, principles, and laws that apply to generalized systems or their subclasses, irrespective of their particular kind, the nature of their component elements, and the relation or 'forces' between them.
System concepts 
- Environment and boundaries
- Systems theory views the world as a complex system of interconnected parts. We scope a system by defining its boundary; this means choosing which entities are inside the system and which are outside – part of the environment. We then make simplified representations (models) of the system in order to understand it and to predict or impact its future behavior. These models may define the structure and/or the behavior of the system.
- Natural and human-made systems
- There are natural and human-made (designed) systems. Natural systems may not have an apparent objective but their outputs can be interpreted as purposes. Human-made systems are made with purposes that are achieved by the delivery of outputs. Their parts must be related; they must be “designed to work as a coherent entity” – else they would be two or more distinct systems.
- Theoretical framework
- An open system exchanges matter and energy with its surroundings. Most systems are open systems; like a car, coffeemaker, or computer. A closed system exchanges energy, but not matter, with its environment; like Earth or the project Biosphere2 or 3. An isolated system exchanges neither matter nor energy with its environment. A theoretical example of such system is the Universe.
- Process and transformation process
- A system can also be viewed as a bounded transformation process, that is, a process or collection of processes that transforms inputs into outputs. Inputs are consumed; outputs are produced. The concept of input and output here is very broad. E.g., an output of a passenger ship is the movement of people from departure to destination.
- A subsystem is a set of elements, which is a system itself, and a component of a larger system.
- System model
- A system comprises multiple views. For the man-made systems it may be such views as planning, requirement (analysis), design, implementation, deployment, structure, behavior, input data, and output data views. A system model is required to describe and represent all these multiple views.
- System architecture
- A system architecture, using one single integrated model for the description of multiple views such as planning, requirement (analysis), design, implementation, deployment, structure, behavior, input data, and output data views, is a kind of system model.
Elements of a system 
Following are considered as the elements of a system in terms of Information systems: –
- Inputs and outputs
- Boundaries and interface
Types of systems 
Systems are classified in different ways:
- Physical or abstract systems.
- Open or closed systems.
- 'Man-made' information systems.
- Formal information systems.
- Informal information systems.
- Computer-based information systems.
- Real-time system.
Physical systems are tangible entities that may be static or dynamic in operation.
An open system has many interfaces with its environment. i.e. system that interacts freely with its environment, taking input and returning output. It permits interaction across its boundary; it receives inputs from and delivers outputs to the outside. A closed system does not interact with the environment; changes in the environment and adaptability are not issues for closed system.
Analysis of systems 
Evidently, there are many types of systems that can be analyzed both quantitatively and qualitatively. For example, with an analysis of urban systems dynamics, [A.W. Steiss]  defines five intersecting systems, including the physical subsystem and behavioral system. For sociological models influenced by systems theory, where Kenneth D. Bailey  defines systems in terms of conceptual, concrete and abstract systems; either isolated, closed, or open, Walter F. Buckley  defines social systems in sociology in terms of mechanical, organic, and process models. Bela H. Banathy  cautions that with any inquiry into a system that understanding the type of system is crucial and defines Natural and Designed systems.
In offering these more global definitions, the author maintains that it is important not to confuse one for the other. The theorist explains that natural systems include sub-atomic systems, living systems, the solar system, the galactic system and the Universe. Designed systems are our creations, our physical structures, hybrid systems which include natural and designed systems, and our conceptual knowledge. The human element of organization and activities are emphasized with their relevant abstract systems and representations. A key consideration in making distinctions among various types of systems is to determine how much freedom the system has to select purpose, goals, methods, tools, etc. and how widely is the freedom to select itself distributed (or concentrated) in the system.
George J. Klir  maintains that no "classification is complete and perfect for all purposes," and defines systems in terms of abstract, real, and conceptual physical systems, bounded and unbounded systems, discrete to continuous, pulse to hybrid systems, et cetera. The interaction between systems and their environments are categorized in terms of relatively closed, and open systems. It seems most unlikely that an absolutely closed system can exist or, if it did, that it could be known by us. Important distinctions have also been made between hard and soft systems. Hard systems are associated with areas such as systems engineering, operations research and quantitative systems analysis. Soft systems are commonly associated with concepts developed by Peter Checkland and Brian Wilson through Soft Systems Methodology (SSM) involving methods such as action research and emphasizing participatory designs. Where hard systems might be identified as more "scientific," the distinction between them is actually often hard to define.
Cultural system 
A cultural system may be defined as the interaction of different elements of culture. While a cultural system is quite different from a social system, sometimes both systems together are referred to as the sociocultural system. A major concern in the social sciences is the problem of order. One way that social order has been theorized is according to the degree of integration of cultural and social factors.
Economic system 
An economic system is a mechanism (social institution) which deals with the production, distribution and consumption of goods and services in a particular society. The economic system is composed of people, institutions and their relationships to resources, such as the convention of property. It addresses the problems of economics, like the allocation and scarcity of resources.
Application of the system concept 
Systems modeling is generally a basic principle in engineering and in social sciences. The system is the representation of the entities under concern. Hence inclusion to or exclusion from system context is dependent of the intention of the modeler.
No model of a system will include all features of the real system of concern, and no model of a system must include all entities belonging to a real system of concern.
Systems in information and computer science 
In computer science and information science, system is a software system which has components as its structure and observable inter-process communications as its behavior. Again, an example will illustrate: There are systems of counting, as with Roman numerals, and various systems for filing papers, or catalogues, and various library systems, of which the Dewey Decimal System is an example. This still fits with the definition of components which are connected together (in this case in order to facilitate the flow of information).
System can also be used referring to a framework, be it software or hardware, designed to allow software programs to run, see platform.
Systems in engineering and physics 
In engineering and physics, a physical system is the portion of the universe that is being studied (of which a thermodynamic system is one major example). Engineering also has the concept of a system that refers to all of the parts and interactions between parts of a complex project. Systems engineering refers to the branch of engineering that studies how this type of system should be planned, designed, implemented, built, and maintained.
Social and cognitive sciences recognize systems in human person models and in human societies. They include human brain functions and human mental processes as well as normative ethics systems and social/cultural behavioral patterns.
In management science, operations research and organizational development (OD), human organizations are viewed as systems (conceptual systems) of interacting components such as subsystems or system aggregates, which are carriers of numerous complex business processes (organizational behaviors) and organizational structures. Organizational development theorist Peter Senge developed the notion of organizations as systems in his book The Fifth Discipline.
Systems thinking is a style of thinking/reasoning and problem solving. It starts from the recognition of system properties in a given problem. It can be a leadership competency. Some people can think globally while acting locally. Such people consider the potential consequences of their decisions on other parts of larger systems. This is also a basis of systemic coaching in psychology.
Organizational theorists such as Margaret Wheatley have also described the workings of organizational systems in new metaphoric contexts, such as quantum physics, chaos theory, and the self-organization of systems.
Systems applied to strategic thinking 
In 1988, military strategist, John A. Warden III introduced his Five Ring System model in his book, The Air Campaign contending that any complex system could be broken down into five concentric rings. Each ring—Leadership, Processes, Infrastructure, Population and Action Units—could be used to isolate key elements of any system that needed change. The model was used effectively by Air Force planners in the First Gulf War. In the late 1990s, Warden applied this five ring model to business strategy.
See also 
- σύστημα, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Library
- 1945, Zu einer allgemeinen Systemlehre, Blätter für deutsche Philosophie, 3/4. (Extract in: Biologia Generalis, 19 (1949), 139–164.
- 1948, Cybernetics: Or the Control and Communication in the Animal and the Machine. Paris, France: Librairie Hermann & Cie, and Cambridge, MA: MIT Press.Cambridge, MA: MIT Press.
- 1956. An Introduction to Cybernetics, Chapman & Hall.
- Steiss 1967, pp. 8–18.
- Bailey, 1994.
- Buckley, 1967.
- Banathy, 1997.
- Klir 1969, pp. 69–72
- Checkland 1997; Flood 1999.
- Warden, John A. III (1988). The Air Campaign: Planning for Combat. Washington, D.C.: National Defense University Press. ISBN 978-1-58348-100-4.
- Warden, John A. III (September 1995). "Chapter 4: Air theory for the 21st century" (in Air and Space Power Journal). Battlefield of the Future: 21st Century Warfare Issues. United States Air Force. Retrieved December 26, 2008.
- Warden, John A. III (1995). "Enemy as a System". Airpower Journal. Spring (9): 40–55. Retrieved 2009-03-25.
- Russell, Leland A.; Warden, John A. (2001). Winning in FastTime: Harness the Competitive Advantage of Prometheus in Business and in Life. Newport Beach, CA: GEO Group Press. ISBN 0-9712697-1-8.
Further reading 
- Alexander Backlund (2000). "The definition of system". In: Kybernetes Vol. 29 nr. 4, pp. 444–451.
- Kenneth D. Bailey (1994). Sociology and the New Systems Theory: Toward a Theoretical Synthesis. New York: State of New York Press.
- Bela H. Banathy (1997). "A Taste of Systemics", ISSS The Primer Project.
- Walter F. Buckley (1967). Sociology and Modern Systems Theory, New Jersey: Englewood Cliffs.
- Peter Checkland (1997). Systems Thinking, Systems Practice. Chichester: John Wiley & Sons, Ltd.
- Robert L. Flood (1999). Rethinking the Fifth Discipline: Learning within the unknowable. London: Routledge.
- George J. Klir (1969). Approach to General Systems Theory, 1969.
- Brian Wilson (1980). Systems: Concepts, methodologies and Applications, John Wiley
- Brian Wilson (2001). Soft Systems Methodology—Conceptual model building and its contribution, J.H.Wiley.
- Beynon-Davies P. (2009). Business Information + Systems. Palgrave, Basingstoke. ISBN 978-0-230-20368-6
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- Definitions of Systems and Models by Michael Pidwirny, 1999–2007.
- Publications with the title "System" (1600–2008) by Roland Müller.
- Definitionen von "System" (1572–2002) by Roland Müller, (most in German).
- Theory and Practical Exercises of System Dynamics by Juan Martin (also in Spanish).