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{{Short description|Family of knowledge representation languages}}
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{{Stack|{{Infobox technology standard
| title             = OWL Web Ontology Language
| status            = Published
| year_started      = 2004
| editors           = Mike Dean ([[BBN Technologies]]), Guus Schreiber
| base_standards    = [[Resource Description Framework]], [[RDFS]]
| related_standards = [[SHACL]]
| abbreviation      = OWL
| domain            = [[Semantic Web]]
| website           = [https://www.w3.org/TR/owl-ref/ OWL Reference]
}}|
{{Infobox technology standard
| title             = OWL 2 Web Ontology Language
| status            = Published
| year_started      = 2009
| editors           = W3C OWL Working Group
| base_standards    = Resource Description Framework, RDFS
| related_standards =
| abbreviation      = OWL 2
| domain            = Semantic Web
| website           = [https://www.w3.org/TR/owl2-overview/ OWL 2 Overview]
}}
}}
The '''Web Ontology Language''' ('''OWL''') is a family of [[Knowledge representation and reasoning|knowledge representation]] languages for authoring [[Ontology (information science)|ontologies]]. Ontologies are a formal way to describe [[Taxonomy|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 (perhaps with monthly revisions) 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.<ref>{{cite web |url=https://www.w3.org/2001/sw/BestPractices/SE/ODSD/ |title=A Semantic Web Primer for Object-Oriented Software Developers |last1=Knublauch |first1=Holger |last2=Oberle |first2=Daniel |last3=Tetlow |first3=Phil |last4=Wallace |first4=Evan |date=March 9, 2006 |website=[[W3C]] |access-date=November 19, 2017}}</ref>

The OWL languages are characterized by [[Semantics (computer science)|formal semantics]]. They are built upon the [[World Wide Web Consortium]]'s (W3C) standard for objects called the [[Resource Description Framework]] (RDF).<ref>{{cite web |url=https://www.w3.org/TR/owl2-overview/ |title=OWL 2 Web Ontology Language Document Overview (Second Edition) |date=11 December 2012 |website=[[W3C]]}}</ref> OWL and RDF have attracted significant academic, medical and commercial interest.

In October 2007,<ref name="timelinehistory">{{cite web|url=http://www.dblab.ntua.gr/~bikakis/XML%20and%20Semantic%20Web%20W3C%20Standards%20Timeline-History.pdf |title=XML and Semantic Web W3C Standards Timeline
}}</ref> a new W3C working group<ref>{{cite web|url=http://www.w3.org/2007/OWL |title=OWL |website=W3.org |access-date=2017-02-23}}</ref> was started to extend OWL with several new features as proposed in the OWL 1.1 member submission.<ref>{{cite web |url=http://www.w3.org/Submission/2006/10/ |title=Submission Request to W3C: OWL 1.1 Web Ontology Language |publisher=W3C |date=2006-12-19 }}</ref> W3C announced the new version of OWL on 27 October 2009.<ref name="w3.org">{{cite web |url=http://www.w3.org/2009/10/owl2-pr |title=W3C Standard Facilitates Data Management and Integration |publisher=W3.org |date=2009-10-27 |access-date=15 October 2013}}</ref> This new version, called OWL 2, soon found its way into semantic editors such as [[Protégé (software)|Protégé]] and [[semantic reasoner]]s such as Pellet,<ref>{{Cite journal | last1 = Sirin | first1 = E. | last2 = Parsia | first2 = B. | last3 = Grau | first3 = B. C. | last4 = Kalyanpur | first4 = A. | last5 = Katz | first5 = Y. | title = Pellet: A practical OWL-DL reasoner | url = http://pellet.owldl.com/papers/sirin05pellet.pdf | doi = 10.1016/j.websem.2007.03.004 | journal = Web Semantics: Science, Services and Agents on the World Wide Web | volume = 5 | issue = 2 | pages = 51–53 | year = 2007 | s2cid = 101226 | archive-url = https://web.archive.org/web/20070627202215/http://pellet.owldl.com/papers/sirin05pellet.pdf | archive-date = 2007-06-27 }}</ref> RacerPro,<ref>{{cite web|url=http://www.racer-systems.com/ |title=RACER - Home |website=Racer-systems.com |access-date=2017-02-23}}</ref> FaCT++<ref>{{Cite book | last1 = Tsarkov | first1 = D. | last2 = Horrocks | first2 = I. | chapter = FaCT++ Description Logic Reasoner: System Description | doi = 10.1007/11814771_26 | chapter-url = http://www.cs.ox.ac.uk/ian.horrocks/Publications/download/2006/TsHo06a.pdf| title = Automated Reasoning | series = Lecture Notes in Computer Science | volume = 4130 | pages = 292–297 | year = 2006 | isbn = 978-3-540-37187-8 | citeseerx = 10.1.1.65.2672 }}</ref><ref>{{cite web|url=http://code.google.com/p/factplusplus/ |title=Google Code Archive - Long-term storage for Google Code Project Hosting |website=Code.google.com |access-date=2017-02-23}}</ref> and HermiT.<ref>{{cite web|url=http://hermit-reasoner.com/ |title=Home |publisher=HermiT Reasoner |access-date=2017-02-23}}</ref>

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 (for example, OWL2 EL). When referring more generally, ''OWL Family'' will be used.<ref>{{cite journal|last=Berners-Lee|first=Tim|author2=James Hendler|author3=Ora Lassila|title=The Semantic Web A new form of Web content that is meaningful to computers will unleash a revolution of new possibilities|journal=Scientific American|date=May 17, 2001|url=http://www.cs.umd.edu/~golbeck/LBSC690/SemanticWeb.html|doi=10.1038/scientificamerican0501-34|volume=284|issue=5|pages=34–43|archive-url=https://web.archive.org/web/20130424071228/http://www.cs.umd.edu/~golbeck/LBSC690/SemanticWeb.html|archive-date=April 24, 2013}}</ref><ref>{{cite book | title=Semantic Web Programming | author=John Hebeler | isbn=978-0-470-41801-7 | date=April 13, 2009 | publisher=Wiley | url-access=registration | url=https://archive.org/details/isbn_9780470418017 }}</ref><ref>{{cite book|last1=Segaran|first1=Toby|last2=Evans|first2=Colin|last3=Taylor|first3=Jamie|title=Programming the Semantic Web|date=July 24, 2009|publisher=O'Reilly Media|isbn=978-0-596-15381-6}}</ref>

==History==
===Early ontology languages===
{{Further|Knowledge representation and reasoning#History}}
There is a long history of [[ontology (information science)|ontological]] development in philosophy and computer science. Since the 1990s, a number of research efforts have explored how the idea of [[knowledge representation]] (KR) from [[artificial intelligence]] (AI) could be made useful on the World Wide Web. These included languages based on [[HTML]] (called [[Simple HTML Ontology Extensions|SHOE]]), based on XML (called XOL, later [[Ontology Inference Layer|OIL]]), 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 [[DARPA Agent Markup Language|DAML]] led by [[James Hendler]].<ref name="book:lacy:owl:c10">{{cite book |title=OWL: Representing Information Using the Web Ontology Language |last=Lacy |first=Lee W. |year=2005 |publisher=Trafford Publishing |location=Victoria, BC |isbn=978-1-4120-3448-7 |chapter=Chapter 10 }}{{self-published source|date=January 2018}}</ref>{{Self-published inline|certain=yes|date=January 2018}}
In March 2001, the ''Joint EU/US Committee on Agent Markup Languages'' decided that DAML should be merged with OIL.<ref name="book:lacy:owl:c10" />
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 (under the DAML program) and the European Union's [[Information Society Technologies]] (IST) funding project. DAML+OIL was intended to be a thin layer above [[RDFS]],<ref name="book:lacy:owl:c10" />
with [[Semantics of programming languages|formal semantics]] based on a [[description logic]] (DL).<ref name="Baader:2005">{{cite book |title=Mechanizing Mathematical Reasoning: Essays in Honor of Jörg H. Siekmann on the Occasion of His 60th Birthday |last1=Baader |first1=Franz |author-link1=Franz Baader |last2=Horrocks |first2=Ian |author-link2=Ian Horrocks |last3=Sattler |first3=Ulrike |author-link3=Ulrike Sattler |year=2005 |publisher=Springer Berlin |location=Heidelberg, DE |isbn=978-3-540-25051-7 |chapter=Description Logics as Ontology Languages for the Semantic Web 
|url=http://www.springerlink.com/content/mf848ceackyx |chapter-url=http://www.springerlink.com/content/axh20n8l34bc3ecb/ |editor1-first=Dieter |editor1-last=Hutter |editor1-link=Dieter Hutter |editor2-first=Werner |editor2-last=Stephan |editor2-link=Werner Stephan}}{{dead link|date=February 2020|bot=medic}}{{cbignore|bot=medic}}</ref>

DAML+OIL is a particularly major influence on OWL; OWL's design was specifically based on DAML+OIL.<ref name=horrocks-et-al>{{cite journal|last1=Horrocks|first1=Ian|last2=Patel-Schneider|first2=Peter F.|last3=van Harmelen|first3=Frank|title=From SHIQ and RDF to OWL: the making of a Web Ontology Language|journal=Web Semantics: Science, Services and Agents on the World Wide Web|volume=1|issue=1|pages=7–26|doi=10.1016/j.websem.2003.07.001|year=2003|citeseerx=10.1.1.2.7039|s2cid=8277015 }}</ref>

===Semantic web standards===
{{Further|Semantic Web}}
{{blockquote|The Semantic Web provides a common framework that allows data to be shared and reused across application, enterprise, and community boundaries.|World Wide Web Consortium|''W3C Semantic Web Activity''<ref name="semantic_web_activity">{{cite web |url=http://www.w3.org/2001/sw/ |title=W3C Semantic Web Activity |author=World Wide Web Consortium |date=2010-02-06 |access-date=18 April 2010 }}</ref>}}

====RDF schema====
{{Further|RDFS}}
{{blockquote|a declarative representation language influenced by ideas from knowledge representation|World Wide Web Consortium|''Metadata Activity''<ref name="metadata.activity">{{cite web |url=http://www.w3.org/Metadata/Activity.html |title=Metadata Activity Statement |author=World Wide Web Consortium |date=2002-08-23 |publisher=World Wide Web Consortium |access-date=20 April 2010 }}</ref>}}

In the late 1990s, the World Wide Web Consortium (W3C) ''Metadata Activity'' started work on [[RDFS|RDF Schema]] (RDFS), a language for [[Resource Description Framework|RDF]] vocabulary sharing. The RDF became a W3C [[web standards|Recommendation]] in February 1999, and RDFS a Candidate Recommendation in March 2000.<ref name="metadata.activity" /> In February 2001, the ''Semantic Web Activity'' replaced the Metadata Activity.<ref name="metadata.activity" /> In 2004 (as part of a wider revision of RDF) RDFS became a  W3C Recommendation.<ref>{{cite web |url=http://www.w3.org/Metadata/Activity.html |title=RDF Vocabulary Description Language 1.0: RDF Schema |author=World Wide Web Consortium |date=2002-08-23 | work=RDF Vocabulary Description Language 1.0 |publisher=World Wide Web Consortium |access-date=20 April 2010 }}</ref>
Though RDFS provides some support for ontology specification, the need for a more expressive ontology language had become clear.<ref name="book:lacy:owl:c9">{{cite book |title=OWL: Representing Information Using the Web Ontology Language |last=Lacy |first=Lee W. |year=2005 |publisher=Trafford Publishing |location=Victoria, BC |isbn=978-1-4120-3448-7 |chapter=Chapter 9 - RDFS}}{{self-published source|date=January 2018}}</ref>{{Self-published inline|certain=yes|date=January 2018}}

====Web-Ontology Working Group====
{{blockquote|As of Monday, the 31st of May, our working group will officially come to an end. We have achieved all that we were chartered to do, and I believe our work is being quite well appreciated.|James Hendler and Guus Schreiber|''Web-Ontology Working Group: Conclusions and Future Work''<ref name="web-ont" />}}

The World Wide Web Consortium (W3C) 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.<ref name="web-ont">{{cite web |url=http://www.w3.org/2001/sw/WebOnt/#L151 |publisher=W3C |title=Web-Ontology (WebOnt) Working Group (Closed) }}</ref> The first working drafts of the [[abstract syntax]], reference and synopsis were published in July 2002.<ref name="web-ont" /> OWL became a formal [[Web standards|W3C recommendation]] on February 10, 2004 and the working group was disbanded on May 31, 2004.<ref name="web-ont" />

====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<ref name="owl1.1">{{cite web |url=http://www.w3.org/Submission/2006/SUBM-owl11-overview-20061219/ |title=OWL 1.1 Web Ontology Language |last1=Patel-Schneider |first1=Peter F. |author-link1=Peter F. Patel-Schneider |last2=Horrocks |first2=Ian |date=2006-12-19 |publisher=World Wide Web Consortium |access-date=26 April 2010 }}</ref> 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.<ref name="grau.2008">{{Cite journal | last1 = Grau | first1 = B. C.  | last2 = Horrocks | first2 = I. | author-link2 = Ian Horrocks| last3 = Motik | first3 = B. | last4 = Parsia | first4 = B. | last5 = Patel-Schneider | first5 = P. F. | last6 = Sattler | first6 = U. | author-link6 = Ulrike Sattler| doi = 10.1016/j.websem.2008.05.001 | title = OWL 2: The next step for OWL | journal = Web Semantics: Science, Services and Agents on the World Wide Web | volume = 6 | issue = 4 | pages = 309–322 | year = 2008 | url = http://www.cs.ox.ac.uk/boris.motik/pubs/ghmppss08next-steps.pdf}}</ref>

OWL 2 became a W3C recommendation in October 2009. OWL 2 introduces profiles to improve scalability in typical applications.<ref name="w3.org" /><ref name="owl2-profiles"/>

===Acronym===
{{blockquote|Why not be inconsistent in at least one aspect of a language which is all about consistency?|Guus Schreiber|''Why OWL and not WOL?''<ref>{{cite web |url=http://www.w3.org/People/Ivan/CorePresentations/RDFTutorial/Slides.html#%28114%29 |title=Why OWL and not WOL? |first=Ivan |last=Herman |author-link=Ivan Herman |work=Tutorial on Semantic Web Technologies |publisher=World Wide Web Consortium |access-date=18 April 2010 }}</ref>}}

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.<ref>{{cite web |url= http://lists.w3.org/Archives/Public/www-webont-wg/2001Dec/0169.html|title=Re: NAME: SWOL versus WOL|work=message sent to W3C webont-wg mailing list on 27 December 2001}}</ref><ref>{{cite web|title=Ontologe Reasoning: The Why and The How|url=http://www2.informatik.hu-berlin.de/~wandelt/SW201213/5OWLDL.pdf|access-date=January 28, 2014|author=Ian Horrocks|author-link=Ian Horrocks|page=7|year=2012}}</ref><ref>{{cite web |url=http://groups.csail.mit.edu/medg/owl-overview.html|title=OWL: the original|access-date=March 22, 2020|date=July 7, 2003}}</ref>

===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 (by syntax). 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.<ref>{{Cite book | last1 = Wang | first1 = T. D. | last2 = Parsia | first2 = B. | last3 = Hendler | first3 = J. | doi = 10.1007/11926078_49 | chapter = A Survey of the Web Ontology Landscape | title = The Semantic Web - ISWC 2006 | series = Lecture Notes in Computer Science | volume = 4273 | page = 682 | year = 2006 | isbn = 978-3-540-49029-6 }}</ref>

==Ontologies==
{{main|Ontology (information science)}}
{{blockquote|An ontology is an explicit specification of a conceptualization.|[[Tom Gruber]]|''  A Translation Approach to Portable Ontology Specifications''<ref name="TRG93">[[Tom Gruber|Gruber, Tom]] (1993); [http://tomgruber.org/writing/ontolingua-kaj-1993.pdf "A Translation Approach to Portable Ontology Specifications"], in ''Knowledge Acquisition'', 5: 199-199</ref>}}

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 [[axiom]]s which place constraints on sets of individuals (called "classes") 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''.<ref name="guide">{{cite web|url=http://www.w3.org/TR/owl-guide/|title=OWL Web Ontology Language Guide|website=[[W3C]]}}</ref>

OWL ontologies can import other ontologies, adding information from the imported ontology to the current ontology.<ref name=horrocks-et-al />

===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:
{| class="wikitable"
!Letter or symbol of logic
!Description
|-
|<math>\mathcal{AL}</math>
|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
|-
|<math>\mathcal{ALC}</math>
|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
|-
|<math>\mathcal{S}</math>
|Adds the definition of the transitivity of a relation to ALC
|-
|<math>\mathcal{H}</math>
|Inclusion and equivalence between relations
|-
|<math>\mathcal{R}</math>
|disjunction of properties, reflexivity, asymmetry, irreflexivity, relations composed of other relations, definition of non-relationship between two individuals
|-
|<math>\mathcal{O}</math> (One of)
|creation of classes via list of all and only the individuals contained
|-
|<math>\mathcal{I}</math> (Reverse)
|definition of inverse property
|-
|<math>\mathcal{F}</math> (Feature)
|definition of functional properties
|-
|<math>\mathcal{N}</math> (Number)
|cardinality restriction: number of elements participating in a certain relationship less than, greater than or equal to a value n
|-
|<math>\mathcal{Q}</math> (Qualified)
|like N, but the relationship can be qualified
|-
|<math>\mathcal{D}_n</math> (Countable domain)
|definition of domains ([[data types]]) to which a relationship can lead (e.g. "Mario is n years old")
|}

==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 (ordered by increasing expressiveness). Each of these [[sublanguage]]s 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 [[Thesaurus|thesauri]] and other [[Taxonomy (general)|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 <math>\mathcal{SHIF}(\mathbf{D})</math>.<ref name="grau.2008" /> 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.<ref name="grau.2008" />

====OWL DL====
OWL DL is designed to provide the maximum expressiveness possible while retaining computational [[Complete theory|completeness]] (either φ or ¬φ holds), [[Decidability (logic)|decidability]] (there is an effective procedure to determine whether φ is derivable or not), and the availability of practical reasoning algorithms. OWL DL includes all OWL language constructs, but they can be used only under certain restrictions (for example, number restrictions may not be placed upon properties which are declared to be transitive; and 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.

This one can be expressed as <math>\mathcal{SHOIN}(\mathbf{D})</math>, using the letters logic above.

====OWL Full====
OWL Full is based on a different semantics from OWL Lite or OWL DL, and was designed to preserve some compatibility with RDF Schema. For example, in OWL Full a class can be treated simultaneously as a collection of individuals and as an individual in its own right; this is not permitted in OWL DL. OWL Full allows an ontology to augment the meaning of the pre-defined (RDF or OWL) vocabulary. OWL Full is undecidable, so no reasoning software is able to perform complete reasoning for it.

===OWL2 profiles===
In OWL2 there are three sublanguages (known as ''profiles''):<ref name="owl2-profiles">{{cite web |url=https://www.w3.org/TR/owl2-profiles/|title=OWL 2 Web Ontology Language Profiles (Second Edition) |date=11 December 2012 |website=[[W3C]]}}</ref>
* OWL2 EL is a fragment that has polynomial time reasoning complexity. It is based on the [[description logic]] <math>\mathcal{EL}</math>.<ref>{{cite conference|last1=Baader|first1=Franz|last2=Brandt|first2=Sebastian|last3=Lutz|first3=Carsten|title=Pushing the <math>\mathcal{EL}</math> envelope|year=2005|publisher=Morgan Kaufmann Publishers Inc.|place =San Francisco, CA, USA|conference=Proceedings of the 19th International Joint Conference on Artificial Intelligence|pages=364–369}}</ref>
* OWL2 QL is designed to enable easier access and query to data stored in databases. It is based on the [[DL-Lite]] family of description logics.<ref>{{cite journal | first1=Diego |last1=Calvanese |first2=Giuseppe |last2=De Giacomo |first3=Domenico |last3=Lembo |first4=Maurizio |last4=Lenzerini |first5=Riccardo |last5=Rosati |title=Tractable Reasoning and Efficient Query Answering in Description Logics: The DL-Lite Family |journal = Journal of Automated Reasoning |volume = 39 |issue = 3 | pages = 385-429 |year=2007 |doi=10.1007/s10817-007-9078-X}}</ref>
* OWL2 RL is a rule subset of OWL 2 (the acronym "RL" stands for "Rule Language"). It is based on the so-called description logic programs (DLP). Later, a logic called <math>\mathcal{RL}</math> corresponding to this profile has been introduced.<ref>{{cite journal |url=https://iccl.inf.tu-dresden.de/w/images/c/cb/TOCL-2011-0035.pdf |title=Complexities of Horn Description Logics |last1=Krötzsch |first1=Markus |last2=Rudolph |first2=Sebastian |last3=Hitzler |first3=Pascal |date=2012 |journal=ACM Transactions on Computational Logic |volume=14 |issue=1 |pages=1–36 |doi=10.1145/2422085.2422087 |access-date=2024-09-23 |name-list-style=vanc}}</ref>

==Syntax==
The OWL family of languages supports a variety of syntaxes. It is useful to distinguish ''high level'' syntaxes aimed at specification from ''exchange'' syntaxes more suitable for general use.

===High level===
These are close to the ontology structure of languages in the OWL family.

====OWL abstract syntax====
High level syntax is used to specify the OWL ontology structure and semantics.<ref name="owl:spec:semantics_and_syntax">{{cite web |url=http://www.w3.org/TR/2004/REC-owl-semantics-20040210/syntax.html |title=OWL Web Ontology Language Semantics and Abstract Syntax |first1=Peter F. |last1=Patel-Schneider |first2=Ian |last2=Horrocks |first3=Hayes |last3=Patrick J. |author-link3=Patrick J. Hayes |date=2004-02-10 |publisher=World Wide Web Consortium |access-date=18 April 2010 }}</ref>

The OWL abstract syntax presents an ontology as a sequence of ''annotations'', ''axioms'' and ''facts''. Annotations carry machine and human oriented meta-data. Information about the classes, properties and individuals that compose the ontology is contained in axioms and facts only.
Each class, property and individual is either ''anonymous'' or identified by an [[URI|URI reference]]. Facts state data either about an individual or about a pair of individual identifiers (that the objects identified are distinct or the same). Axioms specify the characteristics of classes and properties. This style is similar to [[frame language]]s, and quite dissimilar to well known syntaxes for [[description logic|DLs]] and [[Resource Description Framework]] (RDF).<ref name="owl:spec:semantics_and_syntax" />

Sean Bechhofer, ''et al.'' argue that though this syntax is hard to parse, it is quite concrete. They conclude that the name ''abstract syntax'' may be somewhat misleading.<ref>{{cite web |url=http://owl.man.ac.uk/2003/concrete/20031210/ |title=OWL Web Ontology Language Concrete Abstract Syntax |first1=Sean |last1=Bechhofer |author-link1=Sean Bechhofer |first2=Peter F. |last2=Patel-Schneider |first3=Daniele |last3=Turi |author-link3=Daniele Turi |date=2003-12-10 |publisher=[[University of Manchester]] |access-date=18 April 2010 }}</ref>

====OWL2 functional syntax====
This syntax closely follows the structure of an OWL2 ontology. It is used by OWL2 to specify semantics, mappings to exchange syntaxes and profiles.<ref name="owl2:spec:structure">{{cite web |url=http://www.w3.org/TR/2009/REC-owl2-syntax-20091027/ |title= OWL 2 Web Ontology Language Structural Specification and Functional-Style Syntax |last1=Motik |first1=Boris |author-link1=Boris Motik |last2=Patel-Schneider |first2=Peter F. |last3=Parsia |first3=Bijan |author-link3=Bijan Parsia |date=2009-10-27 |work=OWL 2 Web Ontology Language |publisher=World Wide Web Consortium |access-date=18 April 2010 }}</ref>

===Exchange syntaxes===
{{Infobox file format
| name = OWL RDF/XML Serialization
| icon = [[File:XML.svg|100px]]
| extension = .owx, .owl, .rdf
| mime = application/owl+xml, application/rdf+xml<ref>{{cite news |url=http://tools.ietf.org/html/rfc3870 |title=application/rdf+xml Media Type Registration (RFC3870) |author= A. Swartz |newspaper=Ietf Datatracker |author-link= Aaron Swartz |page=2 |publisher=IETF |date= September 2004 |url-status=live |access-date=15 October 2013 |archive-url= https://web.archive.org/web/20130917134108/http://tools.ietf.org/html/rfc3870 |archive-date= 2013-09-17}}</ref>
| owner = [[World Wide Web Consortium]]
| standard = [http://www.w3.org/TR/owl2-xml-serialization/ OWL 2 XML Serialization] {{release date and age|2009|10|27}},<br />[http://www.w3.org/TR/owl-ref/#MIMEType OWL Reference] {{release date and age|2004|02|10}}
| open = Yes
}}

====RDF syntaxes====
Syntactic mappings into [[Resource Description Framework|RDF]] are specified<ref name="owl:spec:semantics_and_syntax" /><ref name="owl2:spec:rdf">{{cite web |url=http://www.w3.org/TR/2009/REC-owl2-mapping-to-rdf-20091027/ |title= OWL 2 Web Ontology Language Mapping to RDF Graphs |last2=Motik |first2=Boris |last1=Patel-Schneider |first1=Peter F. |date=2009-10-27 |work=OWL 2 Web Ontology Language |publisher=World Wide Web Consortium |access-date=18 April 2010 }}</ref>
for languages in the OWL family. Several RDF [[serialization| serialization format]]s have been devised. Each leads to a syntax for languages in the OWL family through this mapping. RDF/XML is normative.<ref name="owl:spec:semantics_and_syntax" /><ref name="owl2:spec:rdf" />

====OWL2 XML syntax====
OWL2 specifies an [[XML]] serialization that closely models the structure of an OWL2 ontology.<ref name="owl2:spec:xml">{{cite web |url=http://www.w3.org/TR/2009/REC-owl2-xml-serialization-20091027/ |title=OWL 2 Web Ontology Language XML Serialization |last1=Motik |first1=Boris |last2=Parsia |first2=Bijan |last3=Patel-Schneider |first3=Peter F. |date=2009-10-27 |work=OWL 2 Web Ontology Language |publisher=World Wide Web Consortium |access-date=18 April 2010 }}</ref>

====Manchester Syntax====
The Manchester Syntax is a compact, human readable syntax with a style close to frame languages.
Variations are available for OWL and OWL2. Not all OWL and OWL2 ontologies can be expressed in this syntax.<ref name="ms:note">{{cite web |url= http://www.w3.org/TR/owl2-manchester-syntax/ |title= OWL 2 Web Ontology Language Manchester Syntax |last1=Horridge |first1=Matthew |author-link1=Matthew Horridge |last2=Patel-Schneider |first2=Peter F. |date=2009-10-27 |work=W3C OWL 2 Web Ontology Language |publisher=World Wide Web Consortium |access-date=18 April 2010 }}</ref>

===Examples===
* The W3C OWL 2 Web Ontology Language provides syntax examples.<ref name="owl2.primer">{{cite web |url=http://www.w3.org/TR/2009/REC-owl2-primer-20091027/ |title=OWL 2 Web Ontology Language Primer |last1=Hitzler |first1=Pascal |author-link1=Pascal Hitzler |last2=Krötzsch |first2=Markus |author-link2=Markus Krötzsch |last3=Parsia |first3=Bijan |last4=Patel-Schneider |first4=Peter F. |last5=Rudolph |first5=Sebastian |author-link5=Sebastian Rudolph |date=2009-10-27 |work=OWL 2 Web Ontology Language |publisher=World Wide Web Consortium |access-date=15 October 2013}}</ref>

====Tea ontology====
Consider an ontology for tea based on a Tea class. First, an ontology identifier is needed. Every OWL ontology must be identified by a [[URI]]{{Citation needed|date=December 2024}} (<nowiki>http://www.example.org/tea.owl</nowiki>, say). This example provides a sense of the syntax. To save space below, preambles and prefix definitions have been skipped.

;OWL2 Functional Syntax: <syntaxhighlight lang="turtle">
Ontology(<http://example.org/tea.owl>
  Declaration( Class( :Tea ) )
)
</syntaxhighlight>

;OWL2 XML Syntax: <syntaxhighlight lang="xml">
 <Ontology ontologyIRI="http://example.org/tea.owl" ...>
   <Prefix name="owl" IRI="http://www.w3.org/2002/07/owl#"/>
   <Declaration>
     <Class IRI="Tea"/>
   </Declaration>
 </Ontology>
</syntaxhighlight>

;Manchester Syntax: <syntaxhighlight lang="turtle">
Ontology: <http://example.org/tea.owl>
Class: Tea
</syntaxhighlight>

;RDF/XML syntax:<syntaxhighlight lang="xml">
<rdf:RDF ...>
    <owl:Ontology rdf:about=""/>
    <owl:Class rdf:about="#Tea"/>
</rdf:RDF>
</syntaxhighlight>

;RDF/[[Turtle (syntax)|Turtle]]: <syntaxhighlight lang="turtle">
 <http://example.org/tea.owl> rdf:type owl:Ontology .
 :Tea  rdf:type            owl:Class .
</syntaxhighlight>

==Semantics==
{{main|Semantics of programming languages}}

===Relation to description logics===
OWL classes correspond to [[description logic]] (DL) ''concepts'', OWL properties to DL ''roles'', while ''individuals'' are called the same way in both the OWL and the DL terminology.<ref>{{cite book |last=Sikos |first=Leslie F. |date=2017 |title=Description Logics in Multimedia Reasoning |url=https://www.springer.com/us/book/9783319540658 |location=Cham |publisher=Springer International Publishing |isbn=978-3-319-54066-5 |doi=10.1007/978-3-319-54066-5 |s2cid=3180114 }}</ref>

{{blockquote|In the beginning, IS-A was quite simple. Today, however, there are almost as many meanings for this inheritance link as there are knowledge-representation systems.|[[Ronald J. Brachman]]|''What IS-A is and isn't''<ref>Brachman, Ronald J. (1983); ''What IS-A is and isn't: An analysis of taxonomic links in semantic networks'', IEEE Computer, vol. 16, no. 10, pp. 30-36</ref>}}

Early attempts to build large ontologies were plagued by a lack of clear definitions. Members of the OWL family have [[model theory|model theoretic]] formal semantics, and so have strong [[logic]]al foundations.

Description logics are a family of logics that are decidable fragments of [[first-order logic]] with attractive and well-understood computational properties. OWL DL and OWL Lite semantics are based on DLs.<ref>{{cite web |url=http://www.cs.man.ac.uk/~horrocks/Publications/download/2003/HoPa03c.pdf |first1=Ian |last1=Horrocks |first2=Peter F. |last2=Patel-Schneider |title=Reducing OWL Entailment to Description Logic Satisfiability }}</ref>
They combine a syntax for describing and exchanging ontologies, and formal semantics that gives them meaning. For example, OWL DL corresponds to the <math>\mathcal{SHOIN}^\mathcal{(D)}</math> description logic, while OWL 2 corresponds to the <math>\mathcal{SROIQ}^\mathcal{(D)}</math> logic.<ref>{{cite book |title=Foundations of Semantic Web Technologies |first1=Pascal |last1=Hitzler  |author-link1=Pascal Hitzler |first2=Markus |last2=Krötzsch |first3=Sebastian |last3=Rudolph |publisher=CRCPress |date=2009-08-25 |isbn=978-1-4200-9050-5 |url=http://www.semantic-web-book.org }}</ref> Sound, complete, terminating [[reasoner]]s (i.e. systems which are guaranteed to derive every consequence of the knowledge in an ontology) exist for these DLs.

===Relation to RDFS===
OWL Full is intended to be compatible with [[RDF Schema]] (RDFS), and to be capable of augmenting the meanings of existing [[Resource Description Framework]] (RDF) vocabulary.<ref>{{cite web |url=http://www.w3.org/TR/2004/REC-owl-features-20040210/ |title=OWL Web Ontology Language Overview |first1=Deborah |last1=McGuinness |author-link=Deborah McGuiness |first2=Frank |last2=van Harmelen |author-link2=Frank van Harmelen |date=2004-02-10 |work=W3C Recommendation for OWL, the Web Ontology Language |publisher=World Wide Web Consortium |access-date=18 April 2010}}</ref> A [[model theory]] describes the formal semantics for
RDF.<ref>{{cite web |url=http://www.w3.org/TR/2004/REC-rdf-mt-20040210/ |title=RDF Semantics |first=Patrick |last=Hayes |author-link=Patrick J. Hayes |date=2004-02-10 |work=Resource Description Framework |publisher=World Wide Web Consortium |access-date=18 April 2010 }}</ref> This interpretation provides the meaning of RDF and RDFS vocabulary. So, the meaning of OWL Full ontologies are defined by extension of the RDFS meaning, and OWL Full is a [[extension (model theory)|semantic extension]] of RDF.<ref>{{cite web |url=http://www.w3.org/TR/owl-semantics/rdfs.html |title=OWL Web Ontology Language Semantics and Abstract Syntax Section 5. RDF-Compatible Model-Theoretic Semantics |first1=Peter F. |last1=Patel-Schneider |first2=Patrick |last2=Hayes |first3=Ian |last3=Horrocks |date=2004-02-10 |work=W3C Recommendation for OWL, the Web Ontology Language |publisher=World Wide Web Consortium |access-date=18 April 2010 }}</ref>

===Open world assumption===
{{blockquote|[The closed] world assumption implies that everything we don't know is ''false'', while the open world assumption states that everything we don't know is ''undefined''.|Stefano Mazzocchi|''Closed World vs. Open World: the First Semantic Web Battle''<ref>{{cite web |url=http://www.betaversion.org/~stefano/linotype/news/91/ |title=Closed World vs. Open World: the First Semantic Web Battle |first=Stefano |last=Mazzocchi |author-link=Setfano Mazzocchi |date=2005-06-16 |access-date=27 April 2010 |archive-url=https://web.archive.org/web/20090624113015/http://www.betaversion.org/~stefano/linotype/news/91/ |archive-date=24 June 2009 }}</ref>}}

The languages in the OWL family use the [[open world assumption]]. Under the open world assumption, if a statement cannot be proven to be true with current knowledge, we cannot draw the conclusion that the statement is false.

====Contrast to other languages====
A [[relational database]] consists of sets of [[tuple]]s with the same [[Attribute (computing)|attribute]]s. [[SQL]] is a query and management language for relational databases. [[Prolog]] is a [[Logic programming|logical programming]] language. Both use the [[closed world assumption]].

==Public ontologies==
===Libraries===
====Biomedical====
* [[Open Biomedical Ontologies|OBO Foundry]]<ref>{{cite web|author=OBO Technical WG |url=http://obofoundry.org |title=The OBO Foundry |publisher=The OBO Foundry |access-date=2017-02-23}}</ref><ref>{{Cite web | url=http://www.berkeleybop.org/ontologies/ | archive-url=https://web.archive.org/web/20070222135936/http://www.berkeleybop.org/ontologies/ | archive-date=2007-02-22 | title=OBO Download Matrix}}</ref>
* [[National Center for Biomedical Ontology|NCBO BioPortal]]<ref>{{cite web |url=http://www.bioontology.org/ncbo/faces/pages/ontology_list.xhtml |title=The National Center for Biomedical Ontology - BioPortal |website=www.bioontology.org |access-date=17 January 2022 |archive-url=https://web.archive.org/web/20070703212612/http://www.bioontology.org/ncbo/faces/pages/ontology_list.xhtml |archive-date=3 July 2007 }}</ref>
* [[National Cancer Institute|NCI]] Enterprise Vocabulary Services

===Standards===
* [[Suggested Upper Merged Ontology]] (SUMO)<ref>{{cite web |url=http://www.ontologyportal.org/translations/SUMO.owl.txt |title=Archived copy |website=www.ontologyportal.org |access-date=17 January 2022 |archive-url=https://web.archive.org/web/20040726184415/http://www.ontologyportal.org/translations/SUMO.owl.txt |archive-date=26 July 2004 }}</ref>
* [[TDWG]]<ref>{{cite web|url=http://community.gbif.org/pg/pages/view/29079/a-review-of-the-tdwg-ontologies |title=GBIF Community Site: Section 1: a review of the TDWG Ontologies |website=Community.gbif.org |date=2013-02-12 |access-date=2017-02-23}}</ref>
* PROV-O,<ref>{{cite web|url=http://www.w3.org/TR/prov-o/ |title=PROV-O: The PROV Ontology |website=W3.org |access-date=2017-02-23}}</ref> the ontology version of the [[World Wide Web Consortium|W3C's]] PROV-DM<ref>{{cite web|url=http://www.w3.org/TR/prov-dm/ |title=PROV-DM: The PROV Data Model |website=W3.org |access-date=2017-02-23}}</ref>
* [[Basic Formal Ontology]] (BFO)<ref>{{Cite web|url=https://www.iso.org/standard/74572.html|title=Iso/Iec 21838-2:2021}}</ref>
* [[European Materials Modelling Ontology]] (EMMO)<ref>{{Cite web|url=https://github.com/emmo-repo/EMMO|title=Elementary Multiperspective Material Ontology (EMMO)|website=[[GitHub]]|date=28 January 2022}}</ref>

===Browsers===
The following tools include public ontology browsers:
* [[Protégé (software)|Protégé OWL]]<ref>{{cite web|url=http://protege.stanford.edu |title=protégé |website=Protege.stanford.edu |access-date=2017-02-23}}</ref>

===Search===
* [[Swoogle]]

==Limitations==
* No direct language support for n-ary relationships. For example, modelers may wish to describe the qualities of a relation, to relate more than 2 individuals or to relate an individual to a list. This cannot be done within OWL. They may need to adopt a pattern instead which encodes the meaning outside the formal semantics.<ref>{{cite web |url=http://www.w3.org/TR/swbp-n-aryRelations/ |title=Defining N-ary Relations on the Semantic Web |first1=Natasha |last1=Noy |author-link=Natasha Noy |first2=Alan |last2=Rector |author-link2=Alan Rector |date=2006-04-12 |publisher=World Wide Web Consortium |access-date=17 April 2010 }}</ref>

==See also==
{{Div col|colwidth=22em}}
* [[Resource Description Framework|RDF]]
* [[Semantic technology]]
* [[Agris: International Information System for the Agricultural Sciences and Technology]]
* [[Common Logic]]
* [[FOAF (software)|FOAF]] + [[wiktionary:DOAC|DOAC]]
* [[Frame language]]
* [[Geopolitical ontology]]
* [[IDEAS Group]]
* [[Meta-Object Facility|Meta-Object Facility (MOF)]], a different standard for the [[Unified Modeling Language|Unified Modeling Language (UML)]] of the [[Object Management Group|Object Management Group (OMG)]]
* [[Metaclass (Semantic Web)]], a featured allowed by OWL to represent knowledge
* [[Multimedia Web Ontology Language]]
* [[Semantic reasoner]]
* [[SKOS]]
* [[SSWAP]]: Simple Semantic Web Architecture and Protocol
* [[SHACL]]: Shapes and Constraints Language for RDF
{{Div col end}}

==References==
{{Reflist|30em}}

==Further reading==
* {{cite web |url=http://www.cs.man.ac.uk/~horrocks/ISWC2003/Tutorial/ |archive-url=https://web.archive.org/web/20170715074737/http://www.cs.man.ac.uk/~horrocks/ISWC2003/Tutorial/ |archive-date=2017-07-15 |title=Tutorial on OWL |last1=Bechhofer |first1=Sean |last2=Horrocks |first2=Ian |last3=Patel-Schneider |first3=Peter F. |year=2021}}
* {{cite web |url=http://www.inf.unibz.it/~franconi/dl/course/ |title=Introduction to Description Logics |first=Enrico |last=Franconi |publisher=Free University of Bolzano |year=2002}}
* {{cite conference|last=Horrocks|first=Ian|year=2010|conference=SemTech 2010|url=http://www.comlab.ox.ac.uk/people/ian.horrocks/Seminars/download/Horrocks_Ian_pt1.pdf|title=Description Logic: A Formal Foundation for Ontology Languages and Tools, Part 1: Languages}}
* {{cite conference|last=Horrocks|first=Ian|year=2010|conference=SemTech 2010|url=http://www.comlab.ox.ac.uk/people/ian.horrocks/Seminars/download/Horrocks_Ian_pt2.pdf|title=Description Logic: A Formal Foundation for Ontology Languages and Tools, Part 2: Tools}}

{{Semantic Web}}
{{W3C Standards}}
{{Data Exchange}}

{{Authority control}}

[[Category:Web Ontology Language| ]]
[[Category:World Wide Web Consortium standards]]
[[Category:Resource Description Framework]]
[[Category:XML-based standards]]
[[Category:Declarative programming languages]]
[[Category:Ontology languages]]
[[Category:Semantic Web]]