OWL Web Ontology Language

W3C Working Draft 31 March 2003

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Deborah L. McGuinness (Knowledge Systems Laboratory, Stanford University) dlm@ksl.stanford.edu
Frank van Harmelen (Vrije Universiteit, Amsterdam) Frank.van.Harmelen@cs.vu.nl


The OWL Web Ontology Language is designed for use by applications that need to process the content of information instead of just presenting information to humans. OWL facilitates greater machine readability of Web content than that supported by XML, RDF, and RDF Schema by providing additional vocabulary along with a formal semantics. OWL has three increasingly-expressive sublanguages: OWL Lite, OWL DL, and OWL Full.

This document is written for readers who want a first impression of the capabilities of OWL. It provides an introduction to OWL by informally describing the features of each of the sublanguages of OWL. Some knowledge of RDF Schema is useful for understanding this document, but not essential. After this document, interested readers may turn to the OWL Guide for a more detailed descriptions and extensive examples on the features of OWL. The normative formal definition of OWL can be found in the OWL Semantics and Abstract Syntax.

Status of this document

This is a Last Call Working Draft. The first release of this document was 29 July 2002 and the Web Ontology Working Group has made its best effort to address comments recieved since then, releasing several drafts and resolving a list of issues meanwhile. The working group seeks confirmation that comments have been addressed to the satisfaction of the community.

This is a non-normative overview of OWL; it does not provide a definitive specification of OWL. The examples and other explanatory material herein are provided to help understand OWL, but may not always provide definitive or complete answers. The normative formal definition of OWL can be found in the OWL Semantics and Abstract Syntax.

Comments on this document are due 9 May 2003. They should be sent to the W3C mailing list public-webont-comments@w3.org (with public archive).

This document has been produced as part of the W3C Semantic Web Activity (Activity Statement). A list of patent disclosures related to this work is maintained by W3C, regardless of whether any such disclosures have been made or not.

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C Recommendations and other technical reports is available at http://www.w3.org/TR/.

Table of contents

  1. Introduction
    1. Document Roadmap
    2. Why OWL?
    3. The three sublanguages of OWL
    4. The structure of this document
  2. Language Synopsis
    1. OWL Lite Synopsis
    2. OWL DL and OWL Full Synopsis
  3. Language Description of OWL Lite
    1. OWL Lite RDF Schema Features
    2. OWL Lite Equality and Inequality
    3. OWL Lite Property Characteristics
    4. OWL Lite Property Type Restrictions
    5. OWL Lite Restricted Cardinality
    6. OWL Lite Class Intersection
    7. OWL Datatypes
    8. OWL Lite Header Information
  4. Incremental Language Description of OWL DL and OWL Full
  5. Summary

  6. Acknowledgements

1. Introduction

This document describes the OWL Web Ontology Language. OWL is intended to be used when the information contained in documents needs to be processed by applications, as opposed to situations where the content only needs to be presented to humans. OWL can be used to explicitly represent the meaning of terms in vocabularies and the relationships between those terms. This representation of terms and their interrelationships is called an ontology. OWL has more facilities for expressing meaning and semantics than XML, RDF, and RDF-S, and thus OWL goes beyond these languages in its ability to represent machine readable content on the Web. OWL is a revision of the DAML+OIL web ontology language incorporating lessons learned from the design and application of DAML+OIL.

1.1 Document Roadmap

The OWL Language is described by a set of documents, each fulfilling a different purpose, and catering for a different audience. The following provides a brief roadmap for navigating through this set of documents:

The suggested reading order of these documents is as given, since they have been listed in increasing degree of technical content.

1.2 Why OWL?

The Semantic Web is a vision for the future of the Web in which information is given explicit meaning, making it easier for machines to automatically process and integrate information available on the Web. The Semantic Web will build on XML's ability to define customized tagging schemes and RDF's flexible approach to representing data. The first level above RDF required for the Semantic Web is an ontology language what can formally describe the meaning of terminology used in Web documents. If machines are expected to perform useful reasoning tasks on these documents, the language must go beyond the basic semantics of RDF Schema. The OWL Use Cases and Requirements Document provides more details on ontologies, motivates the need for a Web Ontology Language in terms of six use cases, and formulates design goals, requirements and objectives for OWL.

OWL has been designed to meet this need for a Web Ontology Language. OWL is part of the growing stack of W3C recommendations related to the Semantic Web.

1.3 The three sublanguages of OWL

OWL provides three increasingly expressive sublanguages designed for use by specific communities of implementers and users.

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.

Ontology developers adopting OWL should consider which sublanguage best suits their needs. The choice between OWL Lite and OWL DL depends on the extent to which users require the more-expressive constructs provided by OWL DL and OWL Full. The choice between OWL DL and OWL Full mainly depends on the extent to which users require the meta-modeling facilities of RDF Schema (e.g. defining classes of classes, or attaching properties to classes). When using OWL Full as compared to OWL DL, reasoning support is less predictable since complete OWL Full implementations do not currently exist.

OWL Full can be viewed as an extension of RDF, while OWL Lite and OWL DL can be viewed as extensions of a restricted view of RDF. Every OWL (Lite, DL, Full) document is an RDF document, and every RDF document is an OWL Full document, but only some RDF documents wll be a legal OWL Lite or OWL DL document.

1.4 The structure of this document

This document first describes the features from OWL Lite, followed by a description from the features that are added in OWL DL and OWL Full (OWL DL and OWL Full contain the same features, but OWL Full is more liberal about how these features can be combined).

2. Language Synopsis

This section provides a quick index to all the language features for OWL Lite, OWL DL, and OWL Full.

In this document, italicized terms are terms in OWL. Prefixes of rdf: or rdfs: are used when terms are already present in RDF or RDF Schema. Otherwise terms are introduced by OWL. Thus, the term rdfs:subPropertyOf indicates that subPropertyOf is already in the rdfs vocabulary (technically : the rdfs namespace). Also, the term Class is more precisely stated as owl:Class and is a term introduced by OWL.

2.1 OWL Lite Synopsis

The list of OWL Lite language constructs is given below.

RDF Schema Features: (In)Equality: Property Characteristics:
Property Type Restrictions:

Class Intersection:
Restricted Cardinality:
Header Information:

2.2 OWL DL and Full Synopsis

The list of OWL DL and OWL Full language constructs that are in addition to those of OWL Lite is given below.

Class Axioms: Boolean Combinations of Class Expressions:
Arbitrary Cardinality: Filler Information:

3. Language Description of OWL Lite

This section provides an informal description of the OWL Lite language features. We do not discuss the specific syntax of these features (see the OWL Reference for definitions). Each language feature is hyperlinked to the appropriate place in the OWL Guide for more examples and guidance on usage.

OWL Lite uses only some of the OWL language features and has has more limitations on the use of the features than OWL DL or OWL Full. ... are also only allowed between named classes, ... Similarly, restrictions In OWL Lite classes can only be defined in terms of named superclasses (superclasses cannot be arbitrary expressions), and only certain kinds of class restrictions can be used. Equivalence between classes and subclass relationships between classes are also only allowed between named classes, and not between arbitrary class expressions. Similarly, restrictions in OWL Lite use only named classes. OWL Lite also has a limited notion of cardinality - the only cardinalities allowed to be explicitly stated are 0 or 1.

3.1 OWL Lite RDF Schema Features

The following OWL Lite features related to RDF Schema are included.

3.2 OWL Lite Equality and Inequality

The following OWL Lite features are related to equality or inequality.

3.3 OWL Lite Property Characteristics

There are special identifiers in OWL Lite that are used to provide information concerning properties and their values.

3.4 OWL Lite Property Type Restriction

OWL Lite allows restrictions to be placed on how properties can be used by instances of a class. The following two restrictions limit which values can be used while the next section's restrictions limit how many values can be used.

3.5 OWL Lite Restricted Cardinality

OWL Lite includes a limited form of cardinality restrictions. OWL (and OWL Lite) cardinality restrictions are referred to as local restrictions, since they are stated on properties with respect to a particular class. That is, the restrictions constrain the cardinality of that property on instances of that class. OWL Lite cardinality restrictions are limited because they only allow statements concerning cardinalities of value 0 or 1 (they do not allow arbitrary values for cardinality, as is the case in OWL DL and OWL Full).

Alternate namings for these restricted forms of cardinality were discussed. Current recommendations are to include any such names in a front end system. More on this topic is available on the publically available webont mail archives with the most relevant message at http://lists.w3.org/Archives/Public/www-webont-wg/2002Oct/0063.html.

3.6 OWL Lite Class Intersection

OWL Lite has contains an intersection constructor but limits its usage.

3.7 Datatypes

OWL uses the RDF mechanisms for data values. See the OWL Guide for a more detailed description.

3.8 OWL Lite Header Information

OWL Lite supports notions of ontology inclusion and relationships and attaching information to ontologies. See the OWL Reference for details and the OWL Guide for examples.

4. Incremental Language Description of OWL DL and OWL FULL

Both OWL DL and OWL Full use the same vocabulary although OWL DL is subject to some restrictions. Roughly, OWL DL requires type separation (a class can not also be an individual or property, a property can not also be an individual or class). This implies that restrictions cannot be applied to the language elements of OWL itself (something that is allowed in OWL Full). Furthermore, OWL DL requires that properties are either ObjectProperties or DatatypeProperties: DatatypeProperties are relations between instances of classes and RDF literals and XML Schema datatypes, while ObjectProperties are relations between instances of two classes. The OWL Abstract Syntax and Semantics document explains the distinctions and limitations. We describe the OWL DL and OWL Full vocabulary that extends the constructions of OWL Lite below.

5. Summary

This document provides an overview of the Web Ontology Language by providing a brief introduction to why one might need a Web ontology language and how OWL fits in with related W3C languages. It also provides a brief description of the three OWL sublanguages: OWL Lite, OWL DL, and OWL Full along with a feature synopsis for each of the languages. This document is an update to the Feature Synopsis Document. It provides simple descriptions of the constructs along with simple examples. It references the OWL reference document, the OWL Guide, and the OWL Abstract Syntax and Semantics document for more details. Previous versions ( January 2, 2003, July 29, 2002, July 8, 2002, June 23, 2002, May 26, 2002, and May 15, 2002) of this document provide the historical view of the evolution of OWL Lite and the issues discussed in its evolution.


This document is the result of extensive discussions within the Web Ontology Working Group as a whole. The members of this working group were Jean-François Baget, James Barnette, Sean Bechhofer, Jonathan Borden, Frederik Brysse, Stephen Buswell, Peter Crowther, Jos De Roo, David De Roure, Mike Dean, Larry Eshelman, Jérôme Euzenat, Dieter Fensel, Tim Finin, Nicholas Gibbins, Pat Hayes, Jeff Heflin, Ziv Hellman, James Hendler, Bernard Horan, Masahiro Hori, Ian Horrocks, Francesco Iannuzzelli, Mario Jeckle, Ruediger Klein, Ora Lassila, Alexander Maedche, Massimo Marchiori, Deborah McGuinness, Libby Miller, Enrico Motta, Leo Obrst, Laurent Olivry , Peter Patel-Schneider, Martin Pike, Marwan Sabbouh, Guus Schreiber, Noboru Shimizu, Michael Sintek, Michael Smith, Ned Smith, John Stanton, Lynn Andrea Stein, Herman ter Horst, Lynne R. Thompson, David Trastour, Frank van Harmelen, Raphael Volz, Evan Wallace, Christopher Welty, and John Yanosy.