A systematic arrangement of all of the important categories of objects or concepts which exist in some field of discourse, showing the relations between them. When complete, an ontology is a categorization of all of the concepts in some field of knowledge, including the objects and all of the properties, relations, and functions needed to define the objects and specify their actions. A simplified ontology may contain only a hierarchical classification (a taxonomy) showing the type subsumption relations between concepts in the field of discourse. An ontology may be visualized as an abstract graph with nodes and labeled arcs representing the objects and relations.
(32NN) A model of how to specify a subject area of interest in which concepts, their attributes, and relationships (associations between concepts) are explicitily named and distinguished. Constraints stipulate distinctions that differentiate concepts, governing how they participate in relationships, and how implicit knowledge is made explicit through qualifiers that allow inferencing over relationships properties. A standard language for representing an ontology on the web is the W3C standard OWL. (329E) See W3C Semantic Web Website (329F)
A vocabulary and set of agreed upon definitions to describe a subject domain. The general subject area of reactive, real-time systems does not have an ontology. However, the various subfields all have their own ontologies. e.g., this glossary. Note that this definition is a good bit narrower than what a philosopher would likely take to be an ontology.