gr. ὄν, ón, ónta (pl.) ou einai: ser, seres. Dupla significação: a) o ser singular, o existente; b) o ato de ser, o fato de ser; e daí: o ser em geral, tomado abstratamente; que pode vir a ser, em Platão  : o Ser em si, a Essência do Ser, Realidade inteligível.

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Plotino   expone las características del Ente to ón por contraposición al mundo sensible en aisthenetói. Dentro de éste juzgamos lo grande pequeño, siendo así que el Ente es por todas partes completamente mejor que lo sensible.[IGAL  ; Plotino - Tratado 22,2 (VI, 4, 2) — O universo sensível se encontra no universo inteligível]

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From the basic directions of meaning of the term οὐσία, we initially choose that which intends beings themselves . In this use, the expression appears. Οὐσίαι is said of various “beings” since they have various being-characters. Beings themselves are always primarily discovered prior to being. Aristotle  , Metaphysics, Book 7, Chapter 2: δοκεῖ δ᾽ ἡ οὐσία ὑπάρχειν φανερώτατα μὲν τοῖς σώμασιν, “the being of beings shows itself openly in σώματα.” [...]

Therefore, for Aristotle and for all research that investigates being, and thereby wants to have a ground to stand on, it is self-evident that it proceeds from the consideration of being (and the being-structure) that is initially there in this manner; that it proceeds from a sense of being that naturalness understands without qualification. Life moves in a natural intelligibility of that which is immediately meant by “being” and “beings” in its speaking. Metaphysics, Book 7, Chapter 3 (end): ὁμολογοῦνται δ᾽ οὐσίαι εἶναι τῶν αἰσθητῶν τινές, “it is agreed that beings in the genuine sense belong to that which is perceived in αἴσθησις.” [Heidegger  , GA18:28]