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Ousia (Οὐσία), is the Ancient Greek noun formed on the feminine present participle for the Greek verb "to be", εἶναι, (such a participle in English, is "being"). It is often translated, perhaps incorrectly, into Latin and English as, substantia and essentia.

Greek philosophers such as Plato and (most importantly) Aristotle used it often in their philosophy; these usages provide the main understanding of its current use, which is mainly in philosophical and theological contexts.

Aristotle used the term in his creation of phyla for animals in biology. Aristotle used ousia and hypostasis in the sense that one, hypostasis, would be a general existence or reality, while the other, ousia, would refer to specific individual things or beings.

Martin Heidegger later maintained that the original meaning of the word was lost in its translation to Latin and subsequently to modern languages. For him it meant "Being" and not "substance"; that is, not some other thing or being that "stood"(-stance) "under"(sub-). He also uses the bi-nomial parousia - apousia to mean 'presence' - 'absence' and the term hypostasis to mean existence.

Origen, (c. 182 – c. 251) used it when he said God is one genus of ousia yet three distinct species of hypostasis: namely the Father, the Son, and the Holy Spirit.

The Synods of Antioch in 264-268 condemned the term homoousios (same substance) because of its Greek language and philosopher roots. The Catholic Encyclopedia article on Paul of Samosata [1] states:

It must be regarded as certain that the council which condemned Paul rejected the term homoousios; but naturally only in a false sense used by Paul; not, it seems because he meant by it an unity of Hypostasis in the Trinity (so St. Hilary), but because he intended by it a common substance out of which both Father and Son proceeded, or which it divided between them, — so St. Basil and St. Athanasius; but the question is not clear. The objectors to the Nicene doctrine in the fourth century made copious use of this disapproval of the Nicene word by a famous council.

Main articles: Homoousian and Chalcedonian

The First Council of Nicaea in 325 debated the terms homoousios and homoiousios. The word homoousios means "same substance", whereas the word homoiousios means "similar substance". The council affirmed the Father, Son, and Holy Spirit (Godhead) are of the homoousious (same substance). Note that the words homoousios and homoiousios differ only by one 'i' (or the Greek letter iota). This is the source of the English idiom "differ not by one iota." Thus, to say two things differ not one iota, is to say that they are the same substance.

The Chalcedonian Creed of 451 stated God is one ousia yet three hypostases.

• Essence
• Substance
• Consubstantial
• Essence-Energies distinction

• Leo Donald Davis, The First Seven Ecumenical Councils (325-787): Their History and Theology, Liturgical Press, 1983. (ISBN 0-8146-5616-1)
• Martin Heidegger, Being and Time.
• Vladimir Lossky The Mystical Theology of the Eastern Church, SVS Press, 1997. (ISBN 0-913836-31-1) James Clarke & Co Ltd, 1991. (ISBN 0-227-67919-9)

• Catholic Encyclopedia: Homoousion
• P.F. Beatrice: The Word "Homoousios" from Hellenism to Christianity
• Schaff's Seven Ecumenical Councils: Excursus on the Word Homousios