I think I maybe understand the question.
Ontology is a philosophical area of study, looking at patterns in the nature of reality and how things can be categorised into hierarchies, what constitutes the existence of a "thing", and so forth. (Read more and, for example, see an interesting ontological question in the form of the Ship of Theseus)
Ontologies are used in Computer Science to document some sort of formally well-defined, unambiguous and agreed upon conceptualisation for a domain being described. Ontologies are machine readable and machine interpretable and allow for reasoning. In the Semantic Web world, Ontologies are defined in OWL. (Read more.)
The previous sense of the term is older and was borrowed by Computer Scientists.
answered 05 Nov '12, 10:59