Glossary of Semantic Web Terms

An ever-evolving glossary of Semantic Web terminology.

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Z

A

anonymous class

An unnamed class. When restrictions are used to describe classes, they actually specify anonymous superclasses of the class being described. For example, we could say that MargheritaPizza is a sub-class of, amongst other things, Pizza and also a subclass of the things that have at least one topping that is MozzarellaTopping. The anonymous class contains all of the individuals that satisfy the restriction.

So, while you may not explicitly define an anonymous class in an ontology, a reasoner may represent anonymous classes based on the restrictions that you have given to other classes.

ARQ

ARQ is a query engine for Jena that supports the SPARQL RDF Query language. SPARQL is the query language developed by the W3C RDF Data Access Working Group.

asserted hierarchy

The 'manually constructed' class hierarchy in an ontology; as opposed to the inferred hierarchy that is generated by a reasoner.

axiom

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C

CamelBack notation

A form of notation which specifies that names should start with a capital letter and should not contain spaces. This form of notation is often recommended for the names of classes in an ontology.

class

A class, in general, is a representation of a concept. It is an abstract representation of some specific classification of things (hence the name class). The name used to identify a class is the perceptual symbol or word used to denote a concept. In an ontology, a class is more specifically a formal definition of a type of information object that may possess a given set of attributes or properties and specific types of relations to other things. The ontology class is the template for an instance or individual of that type. In other words, the class is the schema or model for information of a given type while an instance of the class is considered to be the actual data.

closure axiom

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D

defined class

A class that has at least one set of necessary and sucient conditions is known as a Defined Class; they have a definition, and any individual that satisfies the definition will belong to the class.

Description Logics (DL)

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F

functional property

If a property is functional, for a given individual, there can be at most one individual that is related to the individual via the property. Take, for example, a functional property hasBirthMother. Something can only have one birth mother. If we say that the individual Jean hasBirthMother Peggy and we also say that the individual Jean hasBirthMother Margaret, then because hasBirthMother is a functional property, we can infer that Peggy and Margaret must be the same individual. It should be noted however, that if Peggy and Margaret were explicitly stated to be two dierent individuals then the above statements would lead to an inconsistency.

Functional properties are also known as singles valued properties and also features.

Note also that if a property is functional then it cannot also be transitive.

I

individual

An object or instance of a class.

inferred hierarchy

The class hierarchy constructed by or inferred by a resonaer; as opposed to the asserted hierarchy that is manually created by the author of an ontology.
inverse functional property

If a property is inverse functional then it means that the inverse property is functional. For a given individual, there can be at most one individual related to that individual via the property. Take, for example, the property isBirthMotherOf. This is the inverse property of hasBirthMother — since hasBirthMother is functional, isBirthMotherOf is inverse functional. If we state that Peggy is the birth mother of Jean, and we also state that Margaret is the birth mother of Jean, then we can infer that Peggy and Margaret are the same individual.

inverse property

If some property links individual a to individual b then its inverse property will link individual b to individual a. For example, if a property hasParent is the inverse of a property hasChild then because of the inverse property we can infer that if a given child has a given parent, that parent also has the given child.

O

ontology

An ontology defines the common words and concepts (meanings) used to describe and represent an area of knowledge, and so standardizes the meanings. Ontologies are used by people, databases, and applications that need to share domain information (a domain is just a specific subject area or area of knowledge, like medicine, counter terrorism, imagery, automobile repair, etc.). Ontologies include computer usable definitions of basic concepts in the domain and the relationships among them. They encode knowledge in a domain and also knowledge that spans domains. So, they make that knowledge reusable.

An ontology includes the following:

• Classes (general things) in the many domains of interest
• Instances (particular things)
• Relationships among those things
• Properties (and property values) of those things
• Functions of and processes involving those things
• Constraints on and rules involving those things

open world assumption (a.k.a. open world reasoning)

The open world assumption means that we cannot assume something doesn't exist until it is explicitly stated that it does not exist. In other words, because something hasn't been stated to be true, it cannot be assumed to be false — it is assumed that 'the knowledge just hasn't been added to the knowledge base'.

OWL-DL

OWL-DL is much more expressive than OWL-Lite and is based on Description Logics (hence the suffix DL). Description Logics are a decidable fragment of First Order Logic2 and are therefore amenable to automated reasoning. It is therefore possible to automatically compute the classification hierarchy and check for inconsistencies in an ontology that conforms to OWL-DL.

OWL-Full

OWL-Full is the most expressive OWL sub-language. It is intended to be used in situations where very high expressiveness is more important than being able to guarantee the decidability or computational completeness of the language. It is therefore not possible to perform automated reasoning on OWL-Full ontologies.

OWL-Lite

The syntactically simplest of the three sub-languages of OWL. It is intended to be used in situations where only a simple class hierarchy and simple constraints are needed. For example, it is envisioned that OWL-Lite will provide a quick migration path for existing thesauri and other conceptually simple hierarchies.

OWL - Web Ontology Language

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owl:Thing

The class in an OWL ontology that represents the set containing all individuals. All classes in an OWL ontology are subclasses of owl:Thing.

P

predicate

In an RDF statement (a.k.a. triple), the part that identifies the thing the statement is about is called the subject. The part that identifies the property or characteristic of the subject that the statement specifies is called the predicate, and the part that identifies the value of that property is called the object.

probe class

A class that is added to an ontology for the sole purpose of checking the consistency of an ontology. For example, an ontology author may add a probe class for testing purposes that is deliberately meant to be inconsistent. The author might prefix the class name with "Probe" in order to help other authors understand that it is deliberately meant to be inconsistent; for example: ProbeInconsistentTopping (as a class name in a Pizza ontology).

property

A specific attribute of an individual. Classes can be defined to have properties so that individuals of the class can have values for those properties. OWL properties represent relationships between two individuals. There are two main types of properties:

• Object properties (sometimes also called Resource properties)
  • Object properties link an individual to an individual.
• Datatype properties (sometimes also called Literal properties)
  • Datatype properties link an individual to an XML Schema Datatype value or an rdf literal.

OWL also has a third type of property:

• Annotation properties
  • Annotation properties can be used to add information (metadata — data about data) to classes, individuals and object/datatype properties.

In OWL, properties can have sub properties which specialize their super properties. For example, the sub property hasMother might specialise the more general property of hasParent.

In OWL, a property can be defined as:

• the inverse of another corresponding property,
• functional,
• inverse functional,
• transitive,
• symmetric.

OWL-DL does not allow datatype properties to be transitive, symmetric or have inverse properties.

A property can have a domain and a range.

R

reasoner

One of the main services offered by a reasoner is to test whether or not one class is a subclass of another class. By performing such tests on all of the classes in an ontology it is possible for a reasoner to compute the inferred ontology class hierarchy. Another standard service that is offered by reasoners is consistency checking. Based on the description (conditions) of a class the reasoner can check whether or not it is possible for the class to have any instances. A class is deemed to be inconsistent if it cannot possibly have any instances.

Reasoners are sometimes also known as classifiers.

restriction

In OWL, properties are used to create restrictions. As the name may suggest, restrictions are used to restrict the individuals that belong to a class. Restrictions in OWL fall into three main categories:

• Quantifier Restrictions
• Cardinality Restrictions
• hasValue Restrictions

A restriction actually describes an anonymous class (an unnamed class).

S

SPARQL
The query language for RDF developed by the W3C RDF Data Access Working Group.
See: SPARQL Query Language for RDF

subclass

The child of a parent class. In OWL, subclass means necessary implication. In other words, if Child is a subclass of Person then ALL instances of Child are instances of Person, without exception — if something is a Child then this implies that it is also a Person.

symmetric property

If a property P is symmetric, and the property relates individual a to individual b then individual b is also related to individual a via property P. For example, if the individual Matthew is related to the individual Gemma via the hasSibling property, then we can infer that Gemma must also be related to Matthew via the hasSibling property. In other words, if Matthew has a sibling that is Gemma, then Gemma must have a sibling that is Matthew. Put another way, the property is its own inverse property.

T

taxonomy

A taxonomy is a collection of Controlled Vocabulary terms organized into a hierarchical structure. Each term in a taxonomy is in one or more parent/child (broader/narrower) relationships to other terms in the taxonomy.

thesaurus

controlled vocabulary in which concepts are represented by preferred terms, formally organized so that relationships between the concepts are made explicit. The purpose of a thesaurus is to guide both the indexer and the searcher to select the same preferred term or combination of preferred terms to represent a given subject.

transitive property

If a property is transitive, and the property relates individual a to individual b, and also individual b to individual c, then we can infer that individual a is related to individual c via property P. Take, for example, a transitive property hasAncestor. If the individual Matthew has an ancestor that is Peter, and Peter has an ancestor that is William, then we can infer that Matthew has an ancestor that is William.

Note that if a property is transitive then its inverse property should also be transitive. In some editors, this must be specified manually. However, the reasoner should assume that if a property is transitive, its inverse property is also a transitive.

Note also that if a property is transitive then it cannot be functional.

U

universal restrictions

Universal restrictions are given the symbol ∀. They constrain the relationships along a given property to individuals that are members of a specific class. For example the universal restriction ∀hasTopping MozzarellaTopping describes the individuals all of whose hasTopping relationships are to members of the class MozzarellaTopping — the individuals do not have a hasTopping relationships to individuals that aren't members of the class MozzarellaTopping.

Universal restrictions are also know as All Restrictions.

For a given property, universal restrictions do not specify the existence of a relationship. They merely state that if a relationship exists for the property then it must be to individuals that are members of a specific class.

V

value partition

An ontology design pattern. more definition required