What is an Ontology?

An ontology is a representation of knowledge, generally of a particular subject (domain), written with a standardized, structured syntax. An ontology contains concepts (resources), which serve to characterize the domain.

An ontology can relate resources to other resources, either internally or in other ontologies. Resources can represent the existence of an individual entity ("grannySmithApple445" or "cellPhoneOwnedByJerry"), define subclasses that have a relationship to a class ("grannySmithApple" subclass of "apple", "cellPhone" subclass of "telephone"), and define a class to be associated with a group of entities or subclasses ("fruit," "communication device"). Resources can be associated and defined using relationships. For example, an individual resource is associated with a class ("apple" is a member of "fruit") or a class is associated with an ontology (class "fruit" is described in an ontology called "food").

Ontologies vs. Controlled Vocabularies

A formal specification of a vocabulary can be something we are all familiar with: a plain list of words, a dictionary, a taxonomy, or a thesaurus. Or, it can be a more technical document: an Entity-Relational (ER) diagram, an Object Model in Unified Modeling Language (UML) diagram, or an eXtensible Markup Language (XML) schema. Many other representations are possible for controlled vocabularies.

There are two views on what makes a controlled vocabulary qualify as an ontology. In the first view, simply expressing the vocabulary in an OWL file makes it an ontology, and further subtleties of classification are not important.

In the second view, a controlled vocabulary becomes an ontology when its concepts are defined explicitly and at least some of them are defined as classes. In addition to this requirement, an ontology needs to conform to strict hierarchical subclass relationships between the classes [Gruber, 1993]. The trivial ontologies that simply specify some terms that do not serve any other purpose than naming, to many ontologists do not further the semantic web. Their lack of classes, relations, and properties makes them insufficiently powerful to be designated as ontologies.

Ontologies can include all of the following, but are not required to include them, depending on which perspective from above you adhere to:

  • Classes (general things, types of things)
  • Instances (individual things)
  • Relationships among things
  • Properties of things
  • Functions, processes, constraints, and rules relating to things

Other Considerations

It is worth noting that a science, also called Ontology (denoted here with a capital "O"), has existed for several centuries and has helped to inform current practice in the computer and information sciences, though major differences do exist. Ontology is the study and description of reality, or what can be said to exist, and an attempt to categorize existing things and their relationship to one another. While Ontology seeks to describe every possible thing, ontologists in computer science tend to work in particular knowledge domains, focusing their work on smaller portions of a larger ontological whole.

Some of these terms, particularly 'ontology', have been defined many different ways in different publications. Deborah McGuinness, for example, has proposed that an ontology could be construed as including the entire spectrum of controlled vocabularies. In this guide we use one of the more common definitions, but usage in other papers or contexts may vary. In The Semantic Web, the authors refer to the 'ontology spectrum,' ranging from weak semantic entities like taxonomies, to strong semantic solutions like conceptual models and advanced logics.


  • T. Gruber, A Translation Approach to Portable Ontology Specification., Knowledge Acquisition 5(2), 199-220, 1993.
  • M. C. Daconta, L. J. Obrst, K. T. Smith, The Semantic Web. Wiley Publishing © 2003. (pp 156-158 ff).

Suggested Citation

Graybeal, J. 2011. "What is an Ontology?." In The MMI Guides: Navigating the World of Marine Metadata. http://marinemetadata.org/guides/vocabs/ont/definition. Accessed October 20, 2020.