Ontology (as used in computer science)

(In short) An ontology is a specification of a conceptualization.

An ontology defines the terms used to describe and represent an area of knowledge. Ontologies are used by people, databases, and applications that need to share domain information (a domain is just a specific subject area or area of knowledge, like medicine, tool manufacturing, real estate, automobile repair, financial management, etc.). Ontologies include computer-usable definitions of basic concepts in the domain and the relationships among them (note that here and throughout this document, definition is not used in the technical sense understood by logicians). They encode knowledge in a domain and also knowledge that spans domains. In this way, they make that knowledge reusable. ... (see: source)

See also: http://en.wikipedia.org/wiki/Ontology_%28computer_science%29

and Ontology (as used in philosophy): http://en.wikipedia.org/wiki/Ontology