τί ἦν εἶναι

English (LSJ)

τό, as a Subst.,    Athe thing-in-itself, the essence, the essence of what it is, the being of what it is, essentia, quiddity, the what it was to be (v. εἰμί - sum) F.

Greek Monolingual

(τί ἦν εἶναι, τὸ) Α
αριστοτελικός όρος ο οποίος χρησιμοποιήθηκε για τη δήλωση της φράσης ὄντως ὄν, της πραγματικής υπόστασης, της ουσίας ενός πράγματος.

Wikipedia EN

The concept originates rigorously with Aristotle (although it can also be found in Plato), who used the Greek expression to ti ên einai (τὸ τί ἦν εἶναι, literally meaning "the what it was to be" and corresponding to the scholastic term quiddity) or sometimes the shorter phrase to ti esti (τὸ τί ἐστι, literally meaning "the what it is" and corresponding to the scholastic term haecceity) for the same idea. This phrase presented such difficulties for its Latin translators that they coined the word essentia (English "essence") to represent the whole expression. For Aristotle and his scholastic followers, the notion of essence is closely linked to that of definition (ὁρισμός). Source: Essence; see also Translatum forum