In general, ontology (pronounced ahn-TAH-luh-djee ) is the study or concern about what kinds of things exist - what Entities there are in the universe. It derives from the Greek onto (being) and logia (written or spoken discourse). It is a branch of metaphysics , the study of first principles or the essence of things.
In information technology, an ontology is the working model of entities and interactions in some particular domain of knowledge or practices, such as electronic commerce or "the activity of planning." In artificial intelligence ( AI ), an ontology is, according to Tom Gruber, an AI specialist at Stanford University, "the specification of conceptualizations, used to help programs and humans share knowledge." In this usage, an ontology is a set of concepts - such as things, events, and relations - that are specified in some way (such as specific natural language) in order to create an agreed-upon vocabulary for exchanging information.