As far as I know, Ουσία (existence) is not the word used by Aristotle for Being. Εστί (to exist) is yet another word in similar context. And then there is τo είναι. Some clarification with regard to these words would be appreciated. Thanks.
The term Ουσία derives from a form of the verb εἰμί - I am, but is a different word.
οὐσία < from οὖσα - ἐοῦσα (ὤν < female participle of the present tense of the verb εἰμί - I am)
Ουσία in Ancient Greek meant: a. Property, wealth, b. philosophical essence, substance c. chemical elements
Εστί means to exist, 3rd person present tense of verb εἰμί / I am.
Είναι -> infinitive present tense of verb εἰμί / I am
being qua being -> όντως ον (ὤν < participle of the present tense of the verb εἰμί (I am) )
Τί ην είναι ~ being qua being ("essential being"), -> quidditas
Τόδε τι -> ουσία/ "what is there in a thing" / basic characteristic /what is the essential component of a being, what is crucial component/property/attribute to differentiate one being from another and for a being to stay the same
επιστήμη (science) -> επίσταμαι, I know in depth, I am able, επ + ίσταμαι = to stand above something
Now the "being/είναι and ουσία may seem interchangeable and this because the form "είναι" means what is there indefinitely. (and we can still use the term with this notion synonymous with essence like "the being of man" "To είναι του ανθρώπου"). But there is also an alternative way of usage: the term είναι has taken the form of "what is there now" or what it seems a thing to be, what is its immediate existence and general formation, and this in contrast with ουσία/essence that is the substrate that stays the same regardless the accidentals applied to the being. So the being of a chair is this chair in front of me which may appear black, wooden but the essence of the chair is the piece of furniture we use to sit on regardless the differences (accidentals - συμβεβηκότα) between different forms of these.
Synonymous with ουσία in some contexts is also the term physis (φύση)
In Hegelian logic, the movement of the Being takes three steps: The immediate: Being (Είναι), the mediated (from other thoughts and reflective thinking): Essence (Ουσία), and the speculative-dialectical synthesis of the Being and Essence which is the Notion/Idea.