Web Ontology Language: OWL

The expressivity of RDF and RDF Schema that was described in [12] is deliberately very limited: RDF is (roughly) limited to binary ground predicates, and RDF Schema is (again roughly) limited to a subclass hierarchy and a property hierarchy, with domain and range definitions of these properties. However, the Web Ontology Working Group of W3C identified a number of characteristic use-cases for Ontologies on the Web which would require much more expressiveness than RDF and RDF Schema. A number of research groups in both America and Europe had already identified the need for a more powerful ontology modelling language. This lead to a joint initiative to define a richer language, called DAML+OIL (the name is the join of the names of the American proposal DAML-ONT, and the European language OIL). DAML+OIL in turn was taken as the starting point for the W3C Web Ontology Working Group in defining OWL, the language that is aimed to be the standardised and broadly accepted ontology language of the Semantic Web. In this chapter, we first describe the motivation for OWL in terms of its requirements, and the resulting non-trivial relation with RDF Schema. We then describe the various language elements of OWL in some detail.