Ontologies are specifications of conceptualizations. It is a description of the concepts and relationships that can exist for an agent or a community of agents. The word Ontology, derives from the Greek words onto (being) and logia (written or spoken discourse). In philosophy, an ontology is a hypothesis about the nature of existence, of what types of things exist; ontology as a branch studies such theories. In Artificial Intelligence, ontology is a body of knowledge describing some specific domain, generally common-sense knowledge domain. Tom Gruber, an AI specialist at Stanford University said that, “The specification of conceptualizations, used to help programs and humans share knowledge.” In 1998, Guarino and Giaretta provide the following definition about ontology: “A set of logical axioms designed to account for the intended meaning of a vocabulary.”
Ontology provides an abstract model of some phenomenon in the world by having identified the relevant concepts of that phenomenon and the type of concepts used, and the constraints of their use, are explicitly defined. Also, Ontology should be machine–readable and it captures consensual knowledge, that is, it is not private of some individual, but accepted by a group.
Components of Ontology
Different knowledge representation formalisms share the following minimal set of components in the time of defining ontology:
- Classes: Classes are representing the concepts. Classes in the ontology are usually organised in taxonomies through which inheritance mechanisms can be applied.
- Relations: Relations represent a type of association between concepts of the domain. Ontologies usually contain binary relations. The first argument is known as the domain of the relation, and the second argument is the range. Binary relations are sometimes used to express concept attributes which are usually distinguished from relations because their range is a data type, such as string, numeric, etc.
- Concept: The range of a relation is a concept.·
- Instances: are used to represent elements or individuals in an ontology.
Classification of Ontology
There are several categorizations of ontologies aspect or subject. Guarino proposed an interesting classification on according to their level of dependence on a point of view or task. They are –
- Top–level ontology
- Domain ontology
- Task ontology
- Application ontology
Figure: Classification of Ontology according to Guarino
This type of ontology describes the vocabulary related to a generic domain and specialized the concepts which is introduced in the top-level ontology. Domain ontology represents those concepts which are real or part of the world. Other than Task ontology, Domain ontologies development are the most complex. They are general enough to be required for accomplishing consensus between a wide community of users. Also, they can present an enormous diversity with many different and dynamic domains of knowledge and millions of possible concepts to model. Corcho said that “A domain ontology can be extracted from special purpose encyclopaedias, dictionaries, nomenclatures, taxonomies, handbooks, scientific special languages (say, chemical formulas), specialized KBs, and from experts.” There are several representative ontologies in the domains of e-commerce, medicine, engineering, enterprise and knowledge management.
Task ontologies describe the vocabulary related to a generic task or activity by specializing the top-level ontologies. They are general enough to be required for achieving consensus between a wide community of users as well as they are concrete enough to present an enormous diversity with many different and dynamic domains of knowledge and millions of possible concepts to model.
These ontologies are the most specific ones. Application ontologies have a very narrow context and limited reusability as they depend on the scope and requirements of a specific application. These ontologies are typically developed ad hoc basis by the application designers. Concepts in application ontologies often correspond to roles played by domain entities.