Ousia is the Ancient Greek noun formed on the feminine present participle of it is analogous to the English participle being, and the Greek ontic. Ousia is often translated (sometimes incorrectly) to Latin as substantia and essentia, and to English as substance and essence; and (loosely) also as (contextually) the Latin word accident — [1] which conflicts with the denotation of sumbebekos, given that Aristotle uses sumbebekos in showing that inhuman things (objects) also are substantive.[2]

The Greek philosophers Plato and Aristotle used ousia in their ontologies; their denotations are the contemporary philosophic and theological usages. Aristotle used ousia in creating animal phyla in biology, and hypostasis denoting general existence (reality), and ousia denoting a specific substance, essence, being, person, or thing.

Quite later, the original meaning of the word ousia was lost in its translation to the Latin, and, subsequently, in its translation to modern languages. For him, ousia means Being, not substance, that is, not some thing or some being that "stood"(-stance) "under"(sub-). Moreover, he also uses the bi-nomial parousia-apousia, denoting presence-absence, and hypostasis denoting existence.