Quick Definition: The study of the nature of existence or reality
Ontology is the philosophical study of the nature of being, becoming, existence, or reality, as well as the basic categories of being and their relations. Traditionally listed as a part of the major branch of philosophy known as metaphysics, ontology deals with questions concerning what entities exist or can be said to exist, and how such entities can be grouped, related within a hierarchy, and subdivided according to similarities and differences.
Ontology, in analytic philosophy, concerns the determination whether some categories of being are fundamental and asks in what sense can the items in those categories be said to “be”. It is the inquiry into being in so much as it is being (“being qua being”), or into beings insofar as they exist—and not insofar as (for instance) particular facts can be obtained about them or particular properties belong to them.
Some philosophers, notably of the Platonic school, contend that all nouns (including abstract nouns) refer to existent entities. Other philosophers contend that nouns do not always name entities, but that some provide a kind of shorthand for reference to a collection of either objects or events. In this latter view, mind, instead of referring to an entity, refers to a collection of mental events experienced by a person; society refers to a collection of persons with some shared characteristics, and geometry refers to a collection of a specific kind of intellectual activity. Between these poles of realism and nominalism, stand a variety of other positions; but any ontology must give an account of which words refer to entities, which do not, why, and what categories result.