Ontology is a study of conceptions of reality and the nature of being. In philosophy, ontology (from the Greek nominative : being, genitive : of being (participle of : to be) and -λογία: science, study, theory) is the study of being or existence and forms the basic subject matter of metaphysics. It seeks to describe or posit the basic categories and relationships of being or existence to define entities and types of entities within its framework. It is the science of what is, of the kinds and structures of the objects, properties and relations in every area of reality.

Some philosophers, notably of the Platonic school, contend that all nouns refer to entities. Other philosophers contend that some nouns do not name entities but provide a kind of shorthand way of referring 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 interactions, and geometry refers to a collection of a specific kind of intellectual activity.

As a philosophical subject, ontology chiefly deals with the precise utilization of words as descriptors of entities or realities. Any ontology must give an account of which words refer to entities, which do not, why, and what categories result. When one applies this process to nouns such as electrons, energy, contract, happiness, time, truth, causality, and God, ontology becomes fundamental to many branches of philosophy.