Editing Web Ontology Language (section)

==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")
|}