OWL is an ontology definition language for the web. It comes from the research on the logics of description and is in the form of an extension to the RDFS language. With respect to RDFS, OWL introduces new primitives (allValuesFrom, minCardinality, etc.) to structure in a finer form the vocabulary used for modeling.

An OWL document is used to define individuals, concepts and properties on these concepts. Individuals represent the entities of the world, a concept a set of entities and a property a set of relationships established between entities. As for RDF, OWL introduces a subsumption relation defined between concepts and properties. When it is established between two concepts a and b as a subsumes b, it means that the set of entities represented by b is included in the set of entities associated with a. The OWL language has formal semantics defined in model theory. It gives the mathematical meaning of the definitions of an OWL document and of the subsumption relation that can exist between the defined objects.