ousia
The essential nature or ‘substance’ of God, often as contrasted to the ‘energies’ (external actions and influences) through which he is manifest.
uncountable
We could never know God's ousia, but in order to adapt his indescribable nature to our limited intellect, God communicated to us through his activities in the world.
Essence, being.
uncountable
Where it comes from
Borrowed from Ancient Greek οὐσία (ousía), from οὖσα (oûsa), the feminine present participle of εἰμί (eimí, “I am”).
Adapted from the Wiktionary entry ousia (CC BY-SA 4.0).