Web Ontology Language OWL

The Web Ontology Language OWL extends RDF and RDFS. Its primary aim is to bring the expressive and reasoning power of description logic to the semantic web. Unfortunately, not everything from RDF can be expressed in DL. For example, the classes of classes are not permitted in the (chosen) DL, and some of the triple expressions would have no sense in DL. That is why OWL can be only syntactic extension of RDF/RDFS (note that RDFS is both syntactic and semantic extension of RDF). To partially overcome this problem, and also to allow layering within OWL, three species of OWL are defined.

OWL Lite can be used to express taxonomy and simple constraints, such as 0 and 1 cardinality. It is the simplest OWL language and corresponds to description logic SHIF. OWL DL supports maximum expressiveness while retaining computational completeness and decidability. The DL in the name shows that it is intended to support description logic capabilities. OWL DL corresponds to description logic SHOIN. OWL Full has no expressiveness constraints, but also does not guarantee any computational properties. It is formed by the full OWL vocabulary, but does not no impose any syntactic constrains, so that the full syntactic freedom of RDF can be used.

These three languages are layered in a sense that every legal OWL Lite ontology is a legal OWL DL ontology, every legal OWL DL ontology is a legal OWL Full ontology, every valid OWL Lite conclusion is a valid OWL DL conclusion, and every valid OWL DL conclusion a valid OWL Full conclusion. The inverses of these relations generally do not hold. Also, every OWL ontology is a valid RDF document (i.e., DL expressions are mapped to triples), but not all RDF documents are valid OWL Lite or OWL DL documents. In this text, we are interested primarily in OWL DL. If we will not indicate otherwise, we mean OWL DL by OWL in the rest of the text.