The building blocks of OWL

The Web Ontology Language (OWL) is a component of the Semantic Web used to author ontologies. In simple terms, OWL ontologies are "smart" vocabularies for RDF data. They enable reasoning over the data through a set of explicitly defined rules such as concept hierarchies, disjointness relations, and other types of logical statements.

OWL ontologies are formal knowledge models grounded in strict mathematical logic. Their formal semantics is based on a description logic, which gives them the ability to enrich RDF datasets in a consistent, deterministic, and decidable way.

The main components of OWL

Formally, OWL ontologies are made up of three types of building blocks described below.

Firstly, entities or terms encompass classes, properties, and individuals that make up an ontology and can be thought of as OWL's basic primitives. Examples of entities include ex:Place, ex:friendsWith, ex:TaylorSwift which can be interpreted as a class, property, and individual, respectively.

Secondly, expressions are used to construct various notions and, for example, define classes. In particular, a class expression defines a class by "selecting" individuals that satisfy certain rules.

Finally, axioms or assertions are statements about entities that are asserted to be true. An example of an axiom is the statement asserting that the class ex:SportsEvent is a subclass of the class ex:Event.

Examples of OWL axioms and inferred statements

Below are some examples of OWL axioms and statements that can be derived by means of semantic reasoning.

Subclass hierarchies

A statement of the form

asserts that all the instances of ex:C1 are instances of ex:C2. The ex:rdfs:subClassOf property is transitive, which means that if ex:C1 is a subclass of ex:C2 and ex:C2 is a subclass of ex:C3, then ex:C1 is also a subclass of (contained in) ex:C3.

For example, based on the following ontology:

ex:ElectricSportsCar can be inferred to be a subclass of ex:Car i.e. that all electric sports cars are cars.

Transitivity of custom properties

In OWL, a property is defined as being transitive when it is an instance of the the built-in OWL class owl:TransitiveProperty. Prototypical examples of such properties include "instance of", "located in", "occurred during", and other types of "part of" relations. As an example, the following excerpt

implicitly states that Auckland is located in Oceania.

Class disjointness

A statement of the form

asserts that ex:C1 and ex:C2 have no subclasses or individuals in common. Imposing disjointness constraints is useful for ensuring the consistency of datasets and ontologies themselves, and they can be used to derive multiple conclusions. For example, given the following excerpt:

an OWL reasoner should be able to derive that apples are not vegetables.

Equivalence using owl:sameAs

Statements such as

indicate that two entities refer to the same exact thing. The linked entities can themselves represent classes, properties, or individuals.

For example, the statement

asserts that ex:NewZealand and ex:Aotearoa actually have the same "identity" and can be treated as the aliases of the same entity. This means that any statements made about New Zealand can be combined with those about Aotearoa.

Bottom line

OWL ontologies are used to represent knowledge using explicit computable semantics. Overall, they help improve the quality of data, detect inconsistencies, and discover new relationships. By automatically analysing the data, they facilitate various knowledge management tasks and are a powerful tool in the hands of a knowledge engineer.

See also

• SNOMED CT RF2 to OWL - Convert SNOMED CT RF2 to OWL online.
• OWL reasoning - Using OWL reasoning to infer new knowledge.
• OWL Functional Syntax Parse Tree Viewer - View the parse tree of OWL Functional Syntax.
• The RDF model of the Gene Ontology, demystified - An outline of the structure of the Gene Ontology RDF graph and ways to query it.
• Wikidata's ontology - Inspecting the ontology of Wikidata.