In classical Greek philosophy, ousia (a noun derived from the present participle of the Greek verb "to be") most often expresses one or another of four closely connected concepts: (1) what something is in itself, its being or essence; (2) an entity which is what it is, at least with respect to essential attributes, on its own and without dependence on any more fundamental entity of another type outside itself (in Plato's middle dialogues, the forms; for Aristotle, substance; for the Stoics, the material substrate); (3) for Plato, being as opposed to becoming; and (4) for the Stoics in some instances, existence as opposed to nonexistence. Depending on the context, ousia may be translated as "being," "essence," "reality," or "substance."

Employed in ordinary Greek to speak of a person's wealth and possessions, the word ousia was put to philosophical use by Plato in his early dialogue Euthyphro to state a requirement on...