owl

OWL, or the Web Ontology Language, is a W3C standard for authoring ontologies in the Semantic Web. It provides a formal language for representing rich and complex knowledge about categories, relations, and constraints, and it is designed to be processed by computer programs and reasoning engines. OWL builds on RDF (Resource Description Framework) and RDFS (RDF Schema) and adds more expressive description logics, enabling automated inference such as class membership, property relationships, and consistency checks.

OWL was developed as part of the W3C's Semantic Web initiative. The first version, OWL 1, was published in 2004 with two sublanguages, OWL Lite and OWL DL, and a more permissive OWL Full. In 2009 the W3C released OWL 2, introducing four normative profiles—OWL 2 DL, OWL 2 EL, OWL 2 QL, and OWL 2 RL—designed to balance expressiveness and computability for different applications.

OWL allows constructs such as classes, properties, individuals, equivalence and disjointness, subclass relations, property characteristics (functional, inverse functional, transitive, symmetric), and complex class expressions with intersections, unions, complements, and existential or universal restrictions. Ontologies written in OWL can be reasoned over by description logic reasoners to derive implicit facts, check consistency, and detect inconsistencies.

OWL can be written in several syntaxes, including RDF/XML, Turtle, OWL/XML, and the functional-style syntax. It is typically stored and queried within RDF triple stores and can be integrated with other Semantic Web data via IRIs.

OWL is widely used in biomedicine, enterprise data integration, and knowledge engineering to represent domain knowledge and support automated inference. Popular tools include the Protégé ontology editor and reasoners such as HermiT, Pellet, or Fact++. OWL’s expressive power comes with computational costs, and different profiles trade off decidability and reasoning complexity. OWL 2 DL, EL, QL, and RL are designed to be decidable with reasoners, while OWL Full is not guaranteed decidable and is less suitable for automated reasoning systems.