In its broadest sense, a system is a finite assembly of interdependent things in the universe, separated from its surrounding environment by topological and organizational boundaries, usually one having interconnected components acting as a unitary whole.
Definition and Etymology
In computer science, a system is a group or combination of interrelated, interdependent, or interacting elements forming a collective entity, a set of things working together as a single unit, an assemblage of interrelated elements comprising a unified whole. In other words a system is a group, set or arrangement of units related or connected to form a unity or organic whole. It can also mean an organized set of interrelated elements, ideas or principles. A sub-system is a system which is part of another system. A system typically consists of components (or elements) which are connected together. The term is often used to describe a set of entities which interact with each other.
Etymology: from late Latin systema or Greek σύστημα systema or sustema, which is derived from sunistanai, meaning "to stand together/to place together": sun-/syn- "with/together" +histanai "set up, establish, to cause to stand", see  and . Therefore system means literally a set of components or elements which stand together.
Properties and Components
A systen can have the following properties
- a structure, which can be be static and rigid or flexible and dynamic
- a behavior, if there is an intentional effect on the environment or observable change in the state of the system from the outside
- a history, if the system is dynamic or adaptive and subject to evolution
A system requires at least the following three components or "ingredients":
- elements, units, components, items, entities
- relations, interactions or a form of communication among the elements
- self/non-self boundary or interface to the environment and the surrounding
Forms and Types
A distributed system is a single, unified system of many computers or processors which is distributed over a large geographical area and where the parts are communicating with each other through messages. A complex system is a system whose (emergent) properties are not fully explained by an understanding of its component parts - contrary to a merely complicated system like a mechanical watch which does not have to contain true emergent properties.
A dynamic(al) system has components, processes or flows that change over time. In Mathematics the term is used to describe the time dependence of certain mathematical models, usually models where the evolution or change of the state of a system can be described by differential equations. You can distinguish between linear and non-linear dynamic systems. Non-linear systems are of course much more interesting and show interesting complex structures like strange attractors.
An evolutionary system is based on kind of code and is subject to evolution. This can be for example a Multi-agent system. Multi-agent systems are composed of many interacting, autonomous agents. Adaptive systems are able to adjust themselves to various environments or conditions, and can change themselves to improve and optimize their performance. They are also able to learn from past experiences. A self-organizing system is a system which changes its structure as a function of its experience and as the result of the interplay between environmental influences and autonomous processes.
An open system owns interfaces allowing communication or other interactions with its environment. A closed system does neither communicate nor otherwise interact with its surrondings. Outside events do not influence the self-contained system. An isolated system does not exchange any form of energy, information, matter or material with its environment at all. In general terms, a system has a (more or less permeable or transparent) boundary and can be distinguished from its environment. A general definition of a system is a set of connected elements which influence each other in a specific way which is different from the interactions with or in the environment. In other words different correlations exist between one part of the system and another, than between parts of the system and parts in the environment, or between one part of the environment and another. An autonomous system or agent even needs an environment to complete the perceive-reason-act cycle.
Theories and Examples
Systems Theory is an interdisciplinary field which studies systems and their organization as a whole. It ranges from Cybernetics and Control Theory to Catastrophe Theory, Chaos Theory and Complex Adaptive Systems.
System is like organization a very general and ambiguous term: there are for example central-nervous-systems, endocrine-systems, nintendo-entertainment-systems, computer-systems, operating systems, binary-systems, dynamical-systems, economic-systems, political-systems, writing-systems, coordinate-systems, solar systems, and countless others examples.