System theory glossary

Copyright 2017 Graham Berrisford. One of about 300 papers at Last updated 12/03/2018 10:51



The role of enterprise architects is to observe baseline systems, envisage target systems, and describe both.

So, you might assume their profession uses a controlled vocabulary of system concepts; but not so.

This paper defines the terms that are used, explained and explored in Introducing system terminology and Introducing system ideas.

It adds some more terms discussed in other papers.



Systems. 1

Wholes and parts. 2

System boundary. 2

Interactions between system actors. 3

System structure and behavior 4

System stability and change. 5

Natural systems and designed systems. 5

Social systems, social entities and social cells. 6

Business. 6

Description theory. 6

Questionable concepts and principles. 7




The term system is overloaded with different meanings.

It can refer to a passive structure of connected things and/or an abstract system description.

Here, it usually means a concrete activity system, composed of actors interacting in the activities that characterise the system.


Passive structure: an element or group of related elements that may be acted on, but does not act.

E.g. the Dewey decimal system, the Linnaean classification system, a telephone directory, or a data structure.


Activity system: an island of orderly behavior.

A structure of elements that interact (directly or indirectly) in regular or repeatable behaviors.

The behaviors may maintain or advance the state of the system, consume inputs or deliver outputs.

E.g. the solar system, an organism, a software system, a choir, a tennis match.


Abstract system: a description or model of a concrete system.

E.g. a physical model, a narrative, a context diagram, a network diagram, a causal loop diagram, or a combination of such artifacts.

Abstract descriptions do take concrete forms (they are found in mental and documented models).

At the same time, they describe (model, conceptualise, idealise) a physical reality that can be envisaged or observed as matching the description.


Concrete system: a system that runs in reality.

A realization in physical matter and/or energy of an abstract system description.

The realisation by a real world entity of an abstract system – near enough to satisfy the observer.

One concrete entity can realise different abstract systems.

So, to say an entity is "a system” implies a particular description of the behaviors that characterise it.


The primacy of behavior: the principle that the systems of interest are characterised by their behaviors.
Structures (actors, components, animals or machines) perform those behaviors.
Understanding the structural roles played by actors is part of understanding a system.

The individual actors may come and go; and understanding individual animals or machines is different from understanding a system they play a role in.

Wholes and parts

Holistic view: a description of how parts relate, interact or cooperate in a whole.

E.g. a description of how the muscles of the human heart interact.


Emergent property: a behavior or structure of a whole that depends on interactions between its parts.


Atomic element: a part that is not further divided in a description of a system.

Atomic system actors may be complex entities in their own right, and may play roles in other systems.


Reductionist view: identifying the parts of a whole, naming or describing parts without considering how the parts are related in the whole.


Read Holism and emergent properties for more.


Organicism: the idea that systems are describable at multiple hierarchical levels.

(The term system of systems may mean a nest, a combination or a container of systems.)

System boundary

“Every living organism is essentially an open system. It maintains itself in a continuous inflow and outflow…” Bertalanffy


System environment: the world outside the system of interest.


Closed system: a system that does not interact with anything in the wider environment.

All events of interest are internal to the system.


Open system: a system that interacts with entities and events in a wider environment.

Inputs can change the state of the system; outputs can change the state of entities in the environment.


System boundary: a line (physical or logical) that separates a system from is environment.

It encapsulates the system’s internal structures and behaviors.

It defines the system’s input-process-output boundary.

Inputs and outputs can be flows of material, energy or information.


System interface: a description of inputs and outputs that cross the system boundary.

An interface defines the system as it is seen by external observers.

An interface may be defined in a contract defining services provided or required.

In social and software systems, the primary inputs and outputs are information flows.


Read System boundary for more.

Interactions between system actors

“connected with system theory is… communication. The general notion in communication theory is that of information.” Bertalanffy


Interaction: an exchange of forces, materials or energy or information.


Information: a meaning created or found by an actor in any physical form that acts as a signal.

Any description or direction that has been encoded in a signal or decoded from it by an actor.


Signal: any structure of matter or energy flow in which an actor creates or finds information.

In human communications, the physical forms include brain waves and sound waves.

In digital information systems, the physical form is a data structure in a binary code – in matter or energy.


Information flow (aka message): physically, a signal passed from sender to receiver, logically, a communication.

Information state (aka memory): see “state”.

Information quality: an attribute of information flow or state, such as speed, throughput, availability, security, monetary value.

Information feedback loop: the circular fashion in which system inputs influence future outputs and vice-versa.


Read Communication theory and Information feedback loops for more.


Somewhat related concepts include the following.

Entropy: disorder; across the universe, the total entropy increases with time.

Chaos: unpredictable disorder in how things relate to each other or change over time.

Autopoesis: the process by which organisms build and maintain their bodies from primitive chemicals consumed.

System structure and behavior

“Systems concepts include: system-environment boundary, input, output, process, state….”   Principia Cybernetica


State: the current structure of a thing, as described in the current values of its variable properties.

An entity’s concrete state is directly observable in the values of its physical position, energy and material variables.

E.g. the location, temperature, colour, and matter state (solid/liquid/gas) of a thing you see in front of you.

Information state is found in the values of descriptive variables held in a memory or database of some kind.


(Homeostatic behaviors (maintaining a system’s state variables in a desirable range) may run cyclically or continually.

By contrast, end-to-end behaviors run from start to end, yielding a result or output.

In business systems, these behaviors are usually called value streams, scenarios or processes.

The trigger that starts an activity (or sometimes the activity itself) is often called an event.)


Event: a discrete input that triggers a process that changes a system’s state, depending on the current state.

(Note: this is the basis of modelling a system using discrete event dynamics and System Dynamics.)

(Note: information systems also consume enquiry events that report current state.)


Process: one or more state changes over time, or the logic that determines which state changes lead to which other state changes.

In the abstract, description or specification of how state changes over time, either in discrete steps or continuously.

E.g. a flow chart showing the control logic governing event-triggered activities that result in discrete state changes.

E.g. mathematics describing the continuous change to the position of a planet in its orbit.

A concrete process may be directly observed in changes to the state of the world.

E.g. an apple falls from a tree; a cash payment is handed by one actor to another.


Hysteresis: the process by which a system’s information state can be derived by replaying all events that have so far crossed the system boundary.

Read Determinism and hysteresis for more.

System stability and change

“Cybernetics deals with all forms of behavior in so far as they are regular, or determinate, or reproducible.” Ashby 1956


Deterministic: the quality of a system that means its next state is predictable from its current state and input event.

A deterministic system, in a given state, will respond to a particular stimulus by acting in a predictable way.

Paradoxically, as a result of processing a series of events, a system may change in an unpredictable, chaotic or non-linear fashion.

Read Determinism and hysteresis for more


System state change: a change to the state of a system, which changes the value of at least one variable.


Discrete (or digital) state change: a system’s state advances incrementally in response to discrete events.

Continuous (or analogue) state change: a system’s state advances continually in response to continuous forces or inputs


Linear change: progress or change over time that is represented in a graph as a straight line (or else a regular shape).

Non-linear change: progress or change over time that is represented in a graph as a curve (or else an irregular shape).


System mutation: a change to the nature of a system, which changes the type of at least one variable or behavior.


The following three notions are central to modelling activity systems.

1) The state of an activity system may change.

2) The roles and rules of a system are fixed for a system generation.

3) Changing the roles or rules makes a new system version, or a different system.


Meta system: a system that defines a system or transforms it from one generation to the next.


Read System stability and change for more.

Natural systems and designed systems

Natural system: an entity that behaves as a system before it is perceived to be a system.

Its repeated behaviors are the outcome of evolution rather than the ideas of a living entity.

E.g. The solar system behaved as a system before it was described.


Designed system: an entity that behaves as a system only after it has been conceived by a living entity.

Its reproducible behaviors are a response to the interests or aims of one or more living entities.

E.g. A professional tennis match is conceived and described in the laws of tennis.

Social systems, social entities and social cells

Social entity: a group of actors who may chose their own behaviors, and may interact to reach agreed aims.


Social system: a social entity in which animate actors play roles in regular, repeatable processes.

But the ideal business is a social entity in which actors work happily in the roles of whatever system those actors are hired to work in.


Social cell: a social system whose roles reward the actors of a social entity sufficiently well to ensure the actors voluntarily perpetuate the system.


Read Social cells for discussion of a social cell as mutually beneficial, or insidious, or even as a parasite on society.


Enterprise: has several meanings; usually implies a business led by executive managers/directors, or a segment thereof.


Organization: generally, a structure in which elements are related; here, a structure in which human roles of enterprise are related; it usually implies lines of command and control.


Management system: generally, a system that monitors and directs an entity; here, one that monitors and directs the operational systems of an enterprise.


Team: generally, a group of actors who interact; sometimes a node in an organisation structure; sometimes a group dedicated to the completion of a behavior (a project or task).

Description theory

Thing: a subdivision of the universe, locatable in space and time, describable as instantiating one or more types.


Natural thing: a thing that emerges from the evolution of matter and energy, regardless of description.


Organism: a natural thing whose form is defined by it genes, and engages in the process of Darwinian evolution.


Designed thing: a thing described by an organism before it is created.


Describer: a thing (organism or machine) that can create and use a description.

The survival of describers depends on their ability to create and use descriptions of reality.


Description: a thing that idealises another thing by recording or expressing some of its properties.

A memory, message, model or view that captures/encodes knowledge of a thing’s properties.

It enables that knowledge to remembered and/or communicated.

It enables some questions about the described thing to be answered.


Set: a collection of things that are similar in so far as they instantiate (embody, exemplify) one type.


Type: an intensional definition, composed of property type(s) that describe a thing.


Instance: an embodiment by one thing of a type, giving values to the properties of that type.


Sign: a name, image or effect of a thing or a type, which describers use in recognising, thinking or communicating about that thing or type.


Token: an appearance of a sign or a type in a memory or message.


Read Introducing Description Theory for more.

Questionable concepts and principles

Today’s general system theorist need not embrace all Bertalanffy wrote.

Some ideas von Bertalanffy supposed to be general don’t appear in this update of GST.



Read Hierarchical and network organisations for discussion.


Inexorable progress?

Read Marxism, system theory and EA for discussion.


Goal directedness?

Read Goal-directedness for discussion.



The term implies complicated in some way, but there no agreed measure of complexity.

So, a complex entity may realise a simple system.

Every hamburger is infinitely complex; but the recipe for a hamburger is a relatively simple abstract system description.

Every real-world US government is infinitely complex; but the US constitution is relatively simple abstract system description.

A US government can be called a system where and in so far as it realises the US constitution.

But most of the time, government actors are choosing and performing activities in ad hoc ways not explicitly described in the constitution.

As a social entity, the US government is infinitely complex; as a system it is relatively simple.

Read Complexity for more.


In systems thinking discussions, the following pairs of terms are often confused; but strictly, each pair represents two distinct concepts.


Holistic versus Systemic

Holistic: considering how the parts of a whole are related; taking a view that addresses how parts cooperate to the benefit of the whole.

See also:

Systemic: relating to the whole rather than a part; reaching throughout the whole, pervasive throughout a system.

(E.g. A systemic drug or disease that reaches and has an effect on the whole of a body.)

See also:

To be holistic isn’t the same as being pervasive throughout a system.


Reductionism versus Analysis

Reductionist view: identifying the parts of a whole, naming or describing parts, without considering how the parts are related in the whole.

Analysis: dividing a whole into parts, which can at the same time reveal how the parts are related in the whole.


Analytic(al) versus Systematic

Analytic (aka analytical): using or skilled in using analysis; separating a whole (description or reality) into parts or basic principles.

See also:

Systematic: methodical, acting according to a fixed process or system.

See also:

You may be analytical in a systematic way; but you may be systematic without analysing anything.



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