Elements of an ontology

What is a class concept ?

A class or concept in an ontology is one of the central aspects of an Ontology. Classes describe a certain object of the current domain. Classes can have sub-classes as well as superclasses. Every class has “Thing” as superclass as well as the empty class as subclass. This originates because ontologies are very closely related to mathematical logic and set theory. Classes can have so-called annotations which we will explain below. Annotations basically describe the class as there can be definition annotations or synonym annotations. A class must always have a label annotation, that is the name of the class and how it will be displayed in protege. Classes are hierarchically ordered by the natural order “is subclass of”.

What is an object property ?

The relationship between classes are described with so-called object properties that are used to formulate axioms. These properties are similar to annotations but instead of describing a single class they describe the relation between them. An object property as well as annotation properties can have labels and definition annotations as well. Object properties are a more complex thing since we can assert additional information about properties such as their domain and range, along with defining inverse properties. Moreover, object properties in OWL can have characteristics such as being transitive or symmetric.

What is an axiom ?

An axiom describes the relation between classes and is a powerful way to state a certain property of a class. Axioms consist of a class, an object property and a quantifier. On the other hand, axioms can be used to create a certain class. One could have a class that is defined just by : an individual belongs to that class if and only if it fulfills that particular axiom.

What is an annotation ?

Annotations are the most commonly used properties a class can have. Annotations can describe the class with annotations like definitions, synonyms or seeAlso references.