Although much of the ODP Architecture is concerned with defining formal constructs, the semantics of the architectural model and any modelling languages used have to be described. These concepts are primarily meta-concepts, i.e. concepts which apply generally to any form of modelling activity. It is not intended that these concepts will be formally defined, nor that they be used as the basis of formal definition of other concepts.
Any modelling activity identifies :
a) elements of the universe of discourse;
b) one or more pertinent levels of abstraction.
The elements of the universe of discourse are entities and propositions.
6.1 Entity: Any concrete or abstract thing of interest. While in general the word entity can be used to refer to anything, in the context of modelling it is reserved to refer to things in the universe of discourse being modelled.
6.2 Proposition: An observable fact or state of affairs involving one or more entities, of which it is possible to assert or deny that it holds for those entities.
6.3 Abstraction: The process of suppressing irrelevant detail to establish a simplified model, or the result of that process.
6.4 Atomicity: An entity is atomic at a given level of abstraction if it cannot be subdivided at that level of abstraction.
Fixing a given level of abstraction may involve identifying which elements are atomic.
6.5 System: Something of interest as a whole or as comprised of parts. Therefore a system may be referred to as an entity. A component of a system may itself be a system, in which case it may be called a subsystem.
NOTE - For modelling purposes, the concept of system is understood in its general, system-theoretic sense. The term "system" can refer to an information processing system but can also be applied more generally.
6.6 Architecture (of a system): A set of rules to define the structure of a system and the interrelationships between its parts.