For the Macintosh operating system, which was called System up to version 7.5.5, see Mac OS.
A system is an assemblage of inter-related elements comprising a unified whole. From the Latin and Greek, the term "system" meant to combine, to set up, to place together. A sub-system is a system which is part of another system.
A system typically consists of components (or elements) which are connected together in order to facilitate the flow of information, matter or energy. The term is often used to describe a set of entities which interact, and for which a mathematical model can often be constructed.
Depending on the type of system, a system can often be distinguished from individual machines, elements or processes of that system by the number, arrangements and complexity of those elements. For example, a pulley is a machine, but an elevator, which incorporates pulleys (amongst other components), is a system. Going to the doctor is a process, but health care is a system.
In addition, all so-called "things" (Objects) are actually systems. For example, a cup is an object, but it is also a system for holding hot or cold liquid, or other material. The cup has a certain shape and a handle, it is made of non-porous material and so on, and it is put together in such a way as to provide a useful function. Describing this thing makes up information, and defines a system.
An open system can be influenced by events outside of the declared boundaries of a system. A closed system is self-contained: outside events can have no influence upon the system.
In practice many things are a mixture of the two. For example a prison is a closed system because the prisoners can't get out, and the wardens spend most of their time at the prison. However it is also an open system, because it depends on outside factors and the prisoners and wardens do go outside.
Dynamic systems have components or flows or both, that change over time.
Another distinction is the relation of physical systems to conceptual systems. Physical systems are systems of matter and energy. Conceptual systems are made up of ideas. Conceptual systems generally exist to aid in the accomplishment of specific goals or may be used to model physical systems.
In computer science and information science, system could also be a method or an algorithm. 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).
In operations research and organizational development (OD), organizations are viewed as human systems (conceptual systems) of interacting components such as sub-systems, processes and organizational structures.
Organizational development theorist Peter Senge developed the notion of organizations as systems in his book The Fifth Discipline.
Systems thinking has been identified as an important leadership competency where an individual thinks globally when acting locally. He or she takes into account the potential consequences of a decision on other parts of the organization.