Web Ontology Language (intentionally abbreviated as OWL rather than WOL) is a knowledge representation language used on the Semantic Web, which semantically extends RDF, RDFS, and its predecessor language DAML+OIL. OWL was especially designed for creating web ontologies with a rich set of modeling constructors, addressing the ontology engineering limitations of RDFS. The development of the first version of OWL was started in 2002, and the second version, OWL 2, in 2008. OWL became a W3C Recommendation in 2004, and OWL 2 was standardized in 2009. OWL 2 is a superset of OWL, thus all OWL ontologies can be handled by both OWL and OWL 2 applications. The significant extensions of OWL 2 over OWL include new constructors that improve expressiveness, extended support for datatypes, extended annotation capabilities, syntactic sugar (shorthand notation for common statements), and three profiles (fragments or sublanguages). Each OWL 2 profile provides a different balance between expressive power and reasoning complexity, thereby providing more options for different implementation scenarios:
- OWL 2 EL: particularly designed for handling ontologies with very large numbers of properties and/or classes
- OWL 2 QL: aimed at applications with a very large instance data volume and a priority for query answering
- OWL 2 RL: designed for applications that require scalable reasoning with relatively high expressivity
The default namespace of OWL and OWL 2 is http://www.w3.org/2002/07/owl#, which defines the OWL vocabulary. There is no MIME type defined specifically for OWL, but the application/rdf+xml or the application/xml MIME type is recommended for OWL documents with the .rdf or .owl file extension.
OWL Variants
There are three flavors of OWL, each constituting different compromises between expressivity and computational complexity:
- OWL Full has no restrictions on the use of language constructors. It provides maximum expressiveness, syntactic freedom, but no computational guarantees. The semantics of OWL-Full is a mixture of RDFS and OWL DL (RDF-based semantics).
- OWL DL is a restricted version of OWL Full. OWL-DL provides very high expressiveness, computational completeness (all conclusions are guaranteed to be computable), and decidability (all computations can be finished in finite time). In OWL DL, RDF can be used only for expressing OWL axioms.
- OWL Lite is a subset of OWL DL designed for easy implementation. OWL Lite has limited applicability, because it is suitable only for classification hierarchies and simple constraints.
All these flavors are available in both OWL and OWL 2.