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Ontology (count noun)

1. A particular theory about the nature of being or the kinds of things that have existence (Merriam-Webster online:
2. (mass noun) The branch of metaphysics dealing with the nature of being. (1,
3. A set of concepts and categories in a subject area or domain that shows their properties and the relations between them. (2, (
4. A catalog of the types of things that are assumed to exist in a domain of interest D from the perspective of a person who uses a language L for the purpose of talking about [the domain] D. (P.492, "Knowledge Representation - Logical, Philosophical, and Computational Foundations" by John F. Sowa (2000))
5. The catalogue of concepts (constants, relations, functions, etc.) used to represent knowledge about a problem domain (p.44, "KNOWLEDGE REPRESENTATION AND REASONING" by Ronald J. Brachman and Hector J. Levesque (2004))
6. An ontology is a representational artifact, comprising a taxonomy as proper part, whose representational units are intended to designate some combination of universals, defined classes, and certain relations between them (Smith, B., Kusnierczyk, W., Schober, D., Ceusters, W. Towards a Reference Terminology for Ontology Research and Development in the Biomedical Domain. KR-MED 2006 “Biomedical Ontology in Action”. November 8, 2006, Baltimore, Maryland, USA;
7. An ontology is a logical theory accounting for the intended meaning of a formal vocabulary, i.e. its ontological commitment to a particular conceptualization of the world. The intended models of a logical language using such a vocabulary are constrained by its ontological commitment. An ontology indirectly reflects this commitment (and the underlying conceptualization) by approximating these intended models. (Guarino, N. (1998). Formal ontology and information systems. In Guarino, N., editor, Proceedings of Formal Ontology in Information Systems (FOIS’98), Frontiers in Artificial intelligence and Applications, pages 3-15. Amsterdam: IOS Press. (paper))


  1. TBA. This subsection is intended for Ontology as in philosophy (like the "branch of metaphysics dealing with the nature of being" sort of definitions)


  • In the context of OWL (Web Ontology Language) an ontology is equivalent to a Description Logic knowledge base. (Horrocks, I., Patel-Schneider, P. F., and van Harmelen, F. From SHIQ and RDF to OWL: The making of a web ontology language. Journal of Web Semantics, 2003, 1(1):7. (paper)