Quick Reference Guide

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Document title:
OWL 2 Web Ontology Language
Quick Reference Guide (Second Edition)
Elisa F. Kendall, Sandpiper Software, Inc.
Deborah L. McGuinness, Rensselaer Polytechnic Institute
Evan K. Wallace, NIST
Jie Bao, Rensselaer Polytechnic Institute
Li Ding, Rensselaer Polytechnic Institute
The OWL 2 Web Ontology Language, informally OWL 2, is an ontology language for the Semantic Web with formally defined meaning. OWL 2 ontologies provide classes, properties, individuals, and data values and are stored as Semantic Web documents. OWL 2 ontologies can be used along with information written in RDF, and OWL 2 ontologies themselves are primarily exchanged as RDF documents. The OWL 2 Document Overview describes the overall state of OWL 2, and should be read before other OWL 2 documents.

This document is intended to provide a quick reference to the OWL 2 language, similar to what was provided in the Language Synopsis section of the OWL Web Ontology Language Overview. Inspiration for this effort includes work by the ebiquity Research Group at the University of Maryland Baltimore County (UMBC) on earlier versions of a Reference Card for the Semantic Web.

A current draft printable version is available [1]

Status of this Document
Currently this document is only an editors draft.

Copyright © 2008-2009 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.

[WARNING: This document is currently under revision.]

1 URIs and Namespaces

As described in the Structural Specification and Functional-Style Syntax Specification, OWL ontologies and their elements are defined using International Resource Identifiers (IRIs) RFC-3987. OWL developers may use CURIEs to abbreviate IRIs and thereby improve readability.

The standard namespaces and the respective prefixes used throughout the OWL 2 specifications include:


2 Ontologies

An OWL ontology consists of a set of axioms. It may include annotations and may import other ontologies. An ontology document may also include namespace declarations, including those listed above that contain the reserved vocabulary for the OWL 2 language.

ontologyURI rdf:type owl:Ontology Ontology( ontologyURI ...)

2.1 Importing Ontologies

An ontology can make use of definitions from other ontologies by importing the ontologies that contain them. Importing an ontology means that the user commits to all of the statements made in that ontology.

ontologyURI owl:imports importedOntologyURI Ontology( Import ( importedOntologyURI ) ...)

2.2 Ontology Versioning & Related Annotations

Ontology-specific annotations may include version information or annotations indicating something about compatibility with other versions of the same ontology. For example, an ontology identified by an ontologyURI may also have a versionURI.

ontologyURI owl:versionInfo versionURI Ontology( ontologyURI [ versionURI] ...)

Ontology annotations may provide metadata to facilitate ontology analysis, comparison, mapping, and management. Note that they add no meaningful semantics to the ontology itself, however.

[owl:Ontology -> owl:Ontology]
[owl:Ontology -> owl:Ontology]
[owl:Ontology -> owl:Ontology]

3 Entities

Entities in OWL 2 define the vocabulary of an ontology, and include classes, datatypes, properties, and named individuals.

3.1 Classes

a:Class1 rdf:type owl:Class
a:individual1 rdf:type a:Class1
a:Class1 rdfs:subClassOf a:Class2
Declaration( Class( a:Class1 ) )
ClassAssertion( a:Class1 a:individual1)
SubClassOf( a:Class1 a:Class2 )
a:Class1 is a an OWL class
a:individual1 is an instance of / has type a:Class1
a:Class1 is a subclass of a:Class2

Built-in classes with predefined semantics in OWL include:

  • The class with URI owl:Thing - the set of all individuals, or the top concept
  • The class with URI owl:Nothing - the empty set, or the bottom concept

3.2 Datatypes

3.3 Properties

Object and data properties can be used to represent relationships in the modeled domain; annotation properties can be used to associate nonlogical information with ontologies.

3.3.1 Object Properties

Object properties relate pairs of individuals to one another.

a:property rdf:type owl:ObjectProperty
a:individual1 a:property a:individual2
a:property1 rdfs:subPropertyOf a:property2
Declaration( ObjectProperty( a:property ))
PropertyAssertion( a:property a:individual1 a:individual2)
SubPropertyOf (a:property1 a:property2)
a:property is an ObjectProperty
a:individual1 relates to a:individual2 via property a:property
a:property1 is a subproperty of a:property2

Built-in object properties with predefined semantics in OWL include:

  • The object property with URI owl:TopObjectProperty - connects all possible pairs of individuals, or the top role
  • The object property with URI owl:BottomObjectProperty - does not connect any pair of individuals, or the bottom role

3.3.2 Datatype Properties

Data properties associate individuals with constants.

a:property rdf:type owl:DatatypeProperty
a:individual a:property "constant"
a:property1 rdfs:subPropertyOf a:property2
Declaration( DataProperty( a:property ))
PropertyAssertion( a:property a:individual "constant")
SubPropertyOf (a:property1 a:property2)
a:property is a DataProperty
a:individual has value "constant" for a:property
a:property1 is a subproperty of a:property2

Built-in data properties with predefined semantics in OWL include:

  • The data property with URI owl:TopDataProperty - connects all possible individuals with all possible data values, or the top role
  • The data property with URI owl:BottomDataProperty - does not connect any individuals with any data values, or the bottom role

3.3.3 Annotation Properties

Annotation properties provide a means for augmenting ontologies, entities, or axioms with additional metadata. A number of built-in annotation properties from the RDF Schema vocabulary can be used in OWL. These include:

  • rdfs:label - to associate a human-readable label with an entity
  • rdfs:comment - to associate a comment with an entity
  • rdfs:seeAlso - to associate information available at some URI, or another entity, with an entity to provide additional information
  • rdfs:isDefinedBy - augment the definition of an entity with information available at some URI, or through another entity

In addition, the built-in owl:deprecated property may be used to indicate that a particular entity is deprecated within some context (i.e., set to "true"^^xsd:boolean). See Section 2.2, above, for more on ontology-specific built-in annotations.

The general form for annotation using the built-in vocabulary is as follows:

a:individual rdfs:comment "constant" EntityAnnotation( NamedIndividual( a:individual ) Comment( "constant") )

See Section 8 for more on customizing annotations.

3.4 Individuals

Individuals represent the actual objects in a domain, and may be named or anonymous. Named individuals by definition have URIs associated with them and are therefore considered entities. Examples using named individuals include:

a:individual rdf:type owl:NamedIndividual
a:individual rdf:type a:Class
a:individual a:property "constant"
Declaration( NamedIndividual( a:individual ))
ClassAssertion( a:Class a:individual)
PropertyAssertion( a:property a:individual "constant")
a:individual is a NamedIndividual
a:individual is an instance of a:Class
a:individual has value "constant" for a:property

4 Property Expressions

5 Data Ranges

6 Class Expressions

7 Axioms

8 Annotations

9 Global Restrictions on Axioms

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