Defining Definitions

Ontology can be thought of as  the definition of entities in a domain of interest. It is useful, however, to talk about the word definition in a little more depth as it is a word with a range of usages within the discipline.

Within the OBO community, a definition means a natural language or textual description of that class. The OBO Foundry principles advocate a particular style for these textual definitions of genus and differentia.

One can also have logical definition of a concept. Here, at least in the Web Ontology Language (OWL), definition has a much more precise meaning; that of necessity and sufficiency. A necessary condition is a condition that an instance must fulfill to be a member of that class. Meeting that condition is, however, not enough to recognise that an instance is a member of a class. For example, having an X chromosome is a necessary condition for being a member of the class human male – it is not however sufficient because members of the class human female also have an X chromosome.

A necessary and sufficient condition is one, that when fulfilled, is enough to recognise that an instance is a member of a class. For example, having a Y chromosome is a necessary and sufficient condition for being a member of the class human male.

Necessary and sufficient conditions are also called complete. Classes with only necessary conditions are called partial descriptions.

[Article authored by Robert Stevens and Duncan Hull]