Ontology is the theory or study of being as such; i.e., of the basic characteristics of all reality. Though the term was first coined in the 17th century, ontology is synonymous with metaphysics or "first philosophy" as defined by Aristotle in the 4th century BC. Because metaphysics came to include other studies (e.g., philosophical cosmology and psychology), ontology has become the preferred term for the study of being. It was brought into prominence in the 18th century by Christian Wolff, a German rationalist, for whom it was a deductive discipline leading to necessary truths about the essences of beings. His great successor Immanuel Kant, however, presented influential refutations of ontology as a deductive system and of the ontological argument for God's necessary existence (as supreme and perfect being). With the 20th-century renovation of metaphysics, ontology or ontological thought has again become important, notably among phenomenologists and existentialists, among them Martin Heidegger.
Excerpt from the Encyclopedia Britannica without permission.