Web Ontology Language (OWL): Overview

W3C Working Draft 10 February 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


OWL (the Web Ontology Language) is intended to be used by applications that need to process the content of information instead of just presenting the human-readable content. 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. The OWL language provides 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 Abstract Syntax and Semantics.

Status of this document

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/.

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

This document is a working document for the use by W3C Members and other interested parties. It may be updated, replaced or made obsolete by other documents at any time.

This document has been produced by the Web Ontology Working Group, as part of the W3C Semantic Web Activity. The goals of the Web Ontology working group are discussed in the Web Ontology Working Group charter.

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

There are no patent disclosures related to this work at the time of this writing.

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

1. Introduction

This document describes OWL (the Web Ontology Language). OWL is intended to be used by applications that need to process the document content, as opposed to presenting the content 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 considered 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 next element required for the Semantic Web is a Web Ontology Language (OWL) which can formally describe the meaning of the terminology used in web documents and term interrelationships. 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 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 providing further support for OWL.

OWL is part of a "stack" of Semantic Web related W3C recommendations, in the following way:

provides a surface syntax for structured documents, but imposes no semantic constraints on the meaning of these documents.
XML Schema
is a language for restricting the structure of XML documents.
is a datamodel for objects ("resources") and relations between them, provides a simple semantics for this datamodel, and these datamodels can be represented in an XML syntax.
RDF Schema
is a vocabulary for describing properties and classes of RDF resources, with a semantics for generalization-hierarchies of such properties and classes.
adds more vocabulary for describing properties and classes: among others, relations between classes (e.g. disjointness), cardinality (e.g. "exactly one"), equality, richer typing of properties, characteristics of properties (e.g. symmetry), and enumerated classes.

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 restriction constructs provided by the full language. 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, 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 Full can be viewed as extensions of a restricted view of RDF. Therefore 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 language features from OWL Lite, followed by a description from the language features that are added in OWL DL and OWL Full (OWL DL and OWL Full contain the same language features, but OWL Full is more liberal about how these features can be combined).

2. Language Synopsis

This 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 are 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 are 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 a few limitations w.r.t. OWL DL and OWL Full. Classes can only be defined in terms of named superclasses (superclasses cannot be arbitrary logical expressions), and only certain kinds of class restrictions can be used. Equivalence between classes and subclass relationships between classes are also only allowed to be stated on named classes, and not between arbitrary class expressions. Similarly, property 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 the type of values for a property. The following two restrictions are placed on properties with respect to a class and thus have the impact of limiting the extent of the class with the value restriction.

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 makes use of the RDF datatyping scheme, which provides a mechanism for referring to XML Schema datatypes. Such XML Schema datatypes are identified by a URI, and each time an instance of such a datatype occurs, it must have an RDF attribute rdf:datatype whose value should be the URI reference of the XML Schema datatype. See the OWL Guide for a more detailed description.

3.8 OWL Lite Header Information

OWL supports standard notions of ontology referencing, inclusion, and meta-information. All three levels of OWL include ways of specifying ontologies to import, ontology version information, prior ontology version information, ontologies known to be backward compatible, and ontologies known to be incompatible. See the OWL Reference for definitions.

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 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.