Web Ontology Language

The Web Ontology Language (OWL) is a language for defining and instantiating Web ontologies.^[1] An OWL ontology may include descriptions of classes, along with their related properties and instances. OWL is designed for use by applications that need to process the content of information instead of just presenting information to humans. It facilitates greater machine interpretability of Web content than that supported by XML, RDF, and RDF Schema (RDF-S) by providing additional vocabulary along with a formal semantics. OWL is based on earlier languages OIL and [[DAML+OIL]], and is now a W3C recommendation.

OWL is seen as a major technology for the future implementation of a Semantic Web. It is playing an important role in an increasing number and range of applications, and is the focus of research into tools, reasoning techniques, formal foundations and language extensions.

OWL was designed to provide a common way to process the semantic content of web information. It was developed to augment the facilities for expressing semantics (meaning) provided by XML, RDF, and RDF-S. Consequently, it may be considered an evolution of these web languages in terms of its ability to represent machine-interpretable semantic content on the web. Since OWL is based on XML, OWL information can be easily exchanged between different types of computers using different operating systems, and application languages. Because the language is intended to be read by computer applications, it is sometimes not considered to be human-readable, although this may be a tool issue. OWL is being used to create standards that provide a framework for asset management, enterprise integration, and data sharing on the Web.

An extended version of OWL, (sometimes called OWL 1.1, but with no official status) has been proposed which includes increased expressiveness, a simpler data model and serialization, and a collection of well-defined sub-languages each with known computational properties.

History

A number of research efforts during the mid to late 1990s explored how the idea of knowledge representation (KR) from AI could be made useful on the World Wide Web. These included languages based on HTML (called SHOE), XML (called XOL, later OIL), and various frame-based KR languages and knowledge acquisition approaches.

OWL DL is based in part on the description logic: > $\mathcal{SHOIN} (D)$ , and also on a number of earlier KR systems known as frame-based systems. Its subset OWL Lite is based on the less expressive logic: > $\mathcal{SHIF} (D)$ . All reasoning tasks in both OWL DL and OWL Lite can be reduced to knowledge based (KB) satisfiability. OWL Full operates outside the bounds of description logic, allowing more power and expressivity and having fewer constraints on use, but at the cost of decidability. (OWL Full's semantics is based on the semantics of RDF.) OWL is encoded in RDF/XML documents.^[2]

The OWL Language is a research-based^[3] revision of the DAML+OIL web ontology language. DAML+OIL was developed by a group called the "US/UK ad hoc Joint Working Group on Agent Markup Languages" which was jointly funded by the US Defense Advanced Research Projects Agency (DARPA) under the DAML program and the EU's IST funding project.

The World Wide Web Consortium created the "Web Ontology Working Group" which began work on November 1, 2001 chaired by James Hendler and Guus Schreiber. The first working drafts of the abstract syntax, reference and synopsis were published in July 2002. The OWL documents became a formal W3C recommendation on February 10, 2004 and the working group was disbanded on May 31, 2004.^[4]

Within the working group, effort to identify design goals and requirements was led by Jeff Heflin. The other members of the working group, some with more than a decade of experience designing knowledge-based systems, contributed over 25 use cases, which were later boiled down into defining a set of use cases. ^[5]

Sublanguages

OWL currently has three sublanguages (sometimes also referred to as 'species'): OWL Lite, OWL DL, and OWL Full. These three increasingly expressive sublanguages are designed for use by specific communities of implementers and users.

OWL Lite supports those users primarily needing a classification hierarchy and simple constraints. For example, while it supports cardinality constraints, it only permits cardinality values of 0 or 1. It should be simpler to provide tool support for OWL Lite than its more expressive relatives, and OWL Lite provides a quick migration path for thesauri and other taxonomies. OWL Lite also has a lower formal complexity than OWL DL; see the section on OWL Lite in the OWL Reference for further details.

OWL DL supports those users who want the maximum expressiveness while retaining computational completeness (all conclusions are guaranteed to be computed) and decidability (all computations will finish in finite time). OWL DL includes all OWL language constructs, but they can be used only under certain restrictions (for example, while a class may be a subclass of many classes, a class cannot be an instance of another class). OWL DL is so named due to its correspondence with description logic, a field of research that has studied the logics that form the formal foundation of OWL.

OWL Full is meant for users who want maximum expressiveness and the syntactic freedom of RDF with no computational guarantees. For example, in OWL Full a class can be treated simultaneously as a collection of individuals and as an individual in its own right. OWL Full allows an ontology to augment the meaning of the pre-defined (RDF or OWL) vocabulary. It is unlikely that any reasoning software will be able to support complete reasoning for every feature of OWL Full.

Each of these sublanguages is an extension of its simpler predecessor, both in what can be legally expressed and in what can be validly concluded. The following set of relations hold. Their inverses do not.

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.
Every valid OWL DL conclusion is a valid OWL Full conclusion.

The acronym

Some may claim that the correct acronym for Web Ontology Language should be WOL instead of OWL. Although the character Owl from Winnie the Pooh wrote his name WOL, the acronym OWL was proposed without reference to that character, as an easily pronounced acronym that would yield good logos, suggest wisdom, and honor William A. Martin's One World Language KR project from the 1970s. And, to quote Guus Schreiber, "Why not be inconsistent in at least one aspect of a language which is all about consistency?"

References

^ OWL Web Ontology Language Guide: W3C Recommendation 10 February 2004. W3C (2004-02-10).
^ Ian Horrocks & Peter F. Patel-Schneider. Reducing OWL Entailment to Description Logic Satisfiability.
^ Feature Synopsis for OWL Lite and OWL: W3C Working Draft 29 July 2002. W3C (2002-07-29).
^ Web-Ontology (WebOnt) Working Group (Closed). W3C.
^ OWL Web Ontology Language Use Cases and Requirements: W3C Recommendation 10 February 2004. W3C (2004-02-10).

External links

OWL Web Ontology Language Overview at W3C
OWL Web Ontology Language Guide at W3C
OWL Web Ontology Language Reference at W3C
Visual OWL Visual Modeling Forum page dedicated to graphic notations for OWL.
Tutorial on OWL at the University of Manchester Computer Science Department
OWL 1.1 proposal
Introduction to Description Logics DL course by Enrico Franconi, Faculty of Computer Science, Free University of Bolzano, Italy
Cooperative Ontologies (CO-ODE) web site includes OWL tutorial materials and software.

OWL ontologies

Some existing OWL ontologies may be browsed using an editor such as Protégé-OWL to edit the ontologies posted at the Protégé web site.

There is a large collection of biomedical ontologies available through the OBO Foundry, which are available on their download page, as well a number of others hosted at the NCBO BioPortal.

Other ontologies can be found by searching for appropriate search terms with the filetype set to ".owl" or ".rdf" or by using the Swoogle semantic web search engine.

Standards of the World Wide Web Consortium
Recommendations	CSS · CGI · DOM · HTML · RDF · SVG · SOAP · SMIL · SRGS · SSML · VoiceXML · WSDL · XACML · XHTML · XML · XML Events · XForms · XML Information Set · XML Schema (W3C) · XPath · XQuery · XSL Transformations · XLink · XML Base · SISR · OWL · MathML · RDF Schema · Timed Text ·
Notes	XHTMLplusSMIL
Working Drafts	CURIE · CCXML · InkML · XFrames · XFDL · Web Integration Compound Document · XHTMLplusMathMLplusSVG

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