Web Ontology Language


The Web Ontology Language is a family of knowledge representation languages for authoring ontologies. Ontologies are a formal way to describe taxonomies and classification networks, essentially defining the structure of knowledge for various domains: the nouns representing classes of objects and the verbs representing relations between the objects.
Ontologies resemble class hierarchies in object-oriented programming but there are several critical differences. Class hierarchies are meant to represent structures used in source code that evolve fairly slowly whereas ontologies are meant to represent information on the Internet and are expected to be evolving almost constantly. Similarly, ontologies are typically far more flexible as they are meant to represent information on the Internet coming from all sorts of heterogeneous data sources. Class hierarchies on the other hand tend to be fairly static and rely on far less diverse and more structured sources of data such as corporate databases.
The OWL languages are characterized by formal semantics. They are built upon the World Wide Web Consortium's standard for objects called the Resource Description Framework. OWL and RDF have attracted significant academic, medical and commercial interest.
In October 2007, a new W3C working group was started to extend OWL with several new features as proposed in the OWL 1.1 member submission. W3C announced the new version of OWL on 27 October 2009. This new version, called OWL 2, soon found its way into semantic editors such as Protégé and semantic reasoners such as Pellet, RacerPro, FaCT++ and HermiT.
The OWL family contains many species, serializations, syntaxes and specifications with similar names. OWL and OWL2 are used to refer to the 2004 and 2009 specifications, respectively. Full species names will be used, including specification version. When referring more generally, OWL Family will be used.

History

Early ontology languages

There is a long history of ontological development in philosophy and computer science. Since the 1990s, a number of research efforts have explored how the idea of knowledge representation from artificial intelligence could be made useful on the World Wide Web. These included languages based on HTML, based on XML, and various frame-based KR languages and knowledge acquisition approaches.

Ontology languages for the web

In 2000 in the United States, DARPA started development of DAML led by James Hendler.
In March 2001, the Joint EU/US Committee on Agent Markup Languages decided that DAML should be merged with OIL.
The EU/US ad hoc Joint Working Group on Agent Markup Languages was convened to develop DAML+OIL as a web ontology language. This group was jointly funded by the DARPA and the European Union's Information Society Technologies funding project. DAML+OIL was intended to be a thin layer above RDFS,
with formal semantics based on a description logic.
DAML+OIL is a particularly major influence on OWL; OWL's design was specifically based on DAML+OIL.

Semantic web standards

RDF schema

In the late 1990s, the World Wide Web Consortium Metadata Activity started work on RDF Schema, a language for RDF vocabulary sharing. The RDF became a W3C Recommendation in February 1999, and RDFS a Candidate Recommendation in March 2000. In February 2001, the Semantic Web Activity replaced the Metadata Activity. In 2004 RDFS became a W3C Recommendation.
Though RDFS provides some support for ontology specification, the need for a more expressive ontology language had become clear.

Web-Ontology Working Group

The World Wide Web Consortium created the Web-Ontology Working Group as part of their Semantic Web Activity. It began work on November 1, 2001 with co-chairs James Hendler and Guus Schreiber. The first working drafts of the abstract syntax, reference and synopsis were published in July 2002. OWL became a formal W3C recommendation on February 10, 2004 and the working group was disbanded on May 31, 2004.

OWL Working Group

In 2005, at the OWL Experiences And Directions Workshop a consensus formed that recent advances in description logic would allow a more expressive revision to satisfy user requirements more comprehensively whilst retaining good computational properties.
In December 2006, the OWL1.1 Member Submission was made to the W3C. The W3C chartered the OWL Working Group as part of the Semantic Web Activity in September 2007. In April 2008, this group decided to call this new language OWL2, indicating a substantial revision.
OWL 2 became a W3C recommendation in October 2009. OWL 2 introduces profiles to improve scalability in typical applications.

Acronym

OWL was chosen as an easily pronounced acronym that would yield good logos, suggest wisdom, and honor William A. Martin's One World Language knowledge representation project from the 1970s.

Adoption

A 2006 survey of ontologies available on the web collected 688 OWL ontologies. Of these, 199 were OWL Lite, 149 were OWL DL and 337 OWL Full. They found that 19 ontologies had in excess of 2,000 classes, and that 6 had more than 10,000. The same survey collected 587 RDFS vocabularies.

Ontologies

The data described by an ontology in the OWL family is interpreted as a set of "individuals" and a set of "property assertions" which relate these individuals to each other. An ontology consists of a set of axioms which place constraints on sets of individuals and the types of relationships permitted between them. These axioms provide semantics by allowing systems to infer additional information based on the data explicitly provided. A full introduction to the expressive power of the OWL is provided in the W3C's OWL Guide.
OWL ontologies can import other ontologies, adding information from the imported ontology to the current ontology.

Example

An ontology describing families might include axioms stating that a "hasMother" property is only present between two individuals when "hasParent" is also present, and that individuals of class "HasTypeOBlood" are never related via "hasParent" to members of the "HasTypeABBlood" class. If it is stated that the individual Harriet is related via "hasMother" to the individual Sue, and that Harriet is a member of the "HasTypeOBlood" class, then it can be inferred that Sue is not a member of "HasTypeABBlood". This is, however, only true if the concepts of "Parent" and "Mother" only mean biological parent or mother and not social parent or mother.

Logic

To choose a subset of first-order logic that is decidable, propositional logic was used, increasing its power by adding logics represented by convention with acronyms:
Letter or symbol of logicDescription
Inclusion and equivalence between classes, definition of atomic classes, universe class, intersection between classes, definitions of classes formed by elements that take part in relationships or have a relationship of a certain type only with elements of a certain class, congruence operator between individuals and an individual's membership in a class
adds to AL the empty class, the complement classes, the union of classes and the classes of elements that are in a certain relationship with elements of a certain class
Adds the definition of the transitivity of a relation to ALC
Inclusion and equivalence between relations
disjunction of properties, reflexivity, asymmetry, irreflexivity, relations composed of other relations, definition of non-relationship between two individuals
creation of classes via list of all and only the individuals contained
definition of inverse property
definition of functional properties
cardinality restriction: number of elements participating in a certain relationship less than, greater than or equal to a value n
like N, but the relationship can be qualified
definition of domains to which a relationship can lead

Species

OWL dialects

The W3C-endorsed OWL specification includes the definition of three variants of OWL, with different levels of expressiveness. These are OWL Lite, OWL DL and OWL Full. Each of these sublanguages is a syntactic extension of its simpler predecessor. 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.

    OWL Lite

OWL Lite was originally intended to support 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 was hoped that it would be simpler to provide tool support for OWL Lite than its more expressive relatives, allowing quick migration path for systems using thesauri and other taxonomies. In practice, however, most of the expressiveness constraints placed on OWL Lite amount to little more than syntactic inconveniences: most of the constructs available in OWL DL can be built using complex combinations of OWL Lite features, and is equally expressive as the description logic. Development of OWL Lite tools has thus proven to be almost as difficult as development of tools for OWL DL, and OWL Lite is not widely used.

OWL DL

OWL DL is designed to provide the maximum expressiveness possible while retaining computational completeness, decidability, and the availability of practical reasoning algorithms. OWL DL includes all OWL language constructs, but they can be used only under certain restrictions. 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.
This one can be expressed as, using the letters logic above.