System theory dictionary - and an ontology

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A thing is a part of the universe that can be described as discrete, separable from the rest of the universe.

The things of interest to architects include working systems and system elements.

Architects are part of a meta system whose role is to describe and change systems.

This graphic illustrates the triangular relationship between architects, systems and system descriptions.

Three kinds of thing


<create and use>                 <idealise>

Architects      <observe and envisage>  System elements


It is impossible to impose a single perfect hierarchical structure on all the terms and concepts of interest here.

But people do find it easier to manage things organised in a hierarchical structure.

So, don’t think of the ontological structure below as “right”, it is only a tool to assist understanding.


The ontology / dictionary

System element: a unit of an operational system that can be described.

Structural element: a unit that a system is made of.

Active structural element: a structural element that can be asked to do work

Component: or subsystem, that can perform one or more process steps. It can be related to other components by requesting or delivering services. It can be replaced by any other component with the same interface(s). It is sometimes called a building block.

Interface: a façade component that presents services for access by other components. It encapsulates any internal components needed to deliver the services.

Passive structural element: a structural element that is acted upon.

Object: an item or structure that is used, moved or made. E.g. a data item, data structure, or any kind of structural component.

Location: a place where components and interfaces are found and work is done.

Behavioural element: a unit of what a system does.

Service: the external view of the processing that responds to a singular event or service request.


Process: one or more activities, triggered by an event or service request, that ends by producing a desired effect. It is performed by one or more components. A process is described or defined as a sequence of actions or instructions under a logical control flow.

Adaptation: a process by which an operational system follows rules to act on observations to maintain its state or achieve other desired effects (contrast: evolution)

Description: an abstraction that expresses a thing’s properties by “intension”. (E.g. a name beginning with a “J”.) A description may be a large and complex type


Design: a description in the form of a plan (by drawing or other convention) of a buildable thing, object or system.



Model: a description that abstracts from a referent thing or more elaborate model; it can be a mental, documented, mechanical or other kind of model. It expresses some properties of the referent and enables questions to about it to be answered.

Documented model (artefact): in a readable form (writing or drawing) more stable and shareable than a mental model.


Mental model: that is recorded in a brain, less stable and shareable than a documented model.


Type: a description comprising one or more property types embodied in an observed or envisaged thing, and encoded in a description or model of some kind.

Types are conceived by architects, found in descriptions and embodied in things.

Property (Concept): a type of a thing or element of a thing (a fact, form, function, feature, concept, condition, rule or attribute).

A property type in a description defines the values that a property instance can take (e.g. height in metres, musical pitch).

A property instance is a fact about a particular thing that embodies a property type (e.g. 2, B flat).

Desired effect: a property that describes the result, output or outcome that one or more actors want from a process or operational system.

State: a property comprising the values of an operational system element’s properties at a defined point in time, usually a large and complex type.

Structure: a description that is organised, usually in one of the following ways: a list, a strict hierarchy (a one-to-many cascade with no duplicated items), a redundant hierarchy (with some duplication of items), a grid or matrix (relating element in two lists), or a network (items connected in many-to-many relationships).

Pattern: that can be reused in similar situations to address similar issues, e.g. to organise components so as to cooperate to complete a higher level process.


System description: a description of an operational system. It may map system elements to environmental or contextual elements and enable planning of work to implement the system.



Nomination: definition of a thing by name, or a set of things by listing their names. (E.g. Jack, Jay, Joan and John.)







Meta system element

Meta system role: a role in system architecting.

Architect: one that can create, recall and use descriptions of things that it observes or envisages.


Meta system process: a system architecting activity.

Creation: a process that brings a thing or description into existence, and thus starts its life history.

Observation: a process by which sensors detect things (entities or events) and capture information about them.

Abstraction: a process by which a description is derived either directly from a thing or from a more elaborate or specialised description.

Envisaging: a process by which an architect creates a description of a thing that does not yet exist or is not observed.

Technique: a process that guides the performance of an activity to produce a work product. It may be associated with a pattern.

Evolution: a process of the meta system by which an operational system is changed to produce a new system generation (contrast: adaptation).


Meta system product: an output of system architecting.

See Description and System Element



Below are four words not so far included in the ontological structure above:


defined by genus

and difference



not continuous, dividing a whole or a continuum into distinct or separate things, elements, entities or events.


(aka particular)

[a thing]

that embodies the properties of a type or universal (and perhaps other types).

-          A particular tree is an instance of the type “tree” (and “plant” and “oxygenation engine” and “tall thing”)

-          A particular performance of Hamlet is an instance of “the play Hamlet” (and “entertainment event”).

Operational system:

[a thing]

that is a collection of interrelated components playing roles in processes to maintain system state and/or produce other desired effects.


[a thing]

that is a collection or group of things (set members), definable in one or both ways below.

-          Extensional definition lists all members of a set - by nomination of their names.

-          Intensional definition describes one member of a set - by description of its properties, as in a type elaboration below.

Some overlapping if not identical meanings

Natural language is blessed and cursed with words that have multiple meanings and words that share meanings.

E.g. the words type, concept, property, quality, feature and attribute are all used in describing things.

Some verbs fit some of these words better than others. E.g. You might say a rose bush is a plant that

·         instantiates the type(s) “thorny”, “flowering” and “bushy”

·         embodies the concept(s) “thorny”, “flowering” and “bushy”

·         exhibits the properties “thorny”, “flowering” and “bushy”

·         possesses the qualities, features or attributes “thorny”, “flowering” and “bushy”.


The term "property" is often used ambiguously to mean property type (height) and/or property instance (1.74 metres).

The term "concept" is probably used more often for the property type (height) than the property instance.

A reader suggests all properties are concepts but not all concepts are properties, but it is difficult to find convincing examples of the latter.



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