Ngtjyty

Almost every verb has a noun that implies "the act and effect of" whatever the verb is. So, an existence is the act of existing. Nonetheless, the most simple verb, to be seems to lack such a noun, at least in Spanish (link in Spanish, sorry).

But then someone came up with essence as a possibility for the noun of to be, but in Spanish it lacks that meaning (it means roughly "the most important and charasteristic part of a thing"). But what about Latin? In Latin we have exsistĕre and its noun exsistentia, and also essentĭa which seems to mean the being or essence of a thing, according to the Lewis & Short dictionary.

Can then essentĭa be understood as "the act of being" the same way as exsistentia is "the act of existing"?

No. Essentia (to-be-ness) was coined by Cicero as a Latin counterpart for the Greek οὐσία (ousia). Both words refer to something's essence or nature, that which makes a thing "to be" the particular kind of thing that it is. Nature or essence is contrasted with accidents, those qualities of a thing that can change without changing what kind of thing something is.

Eventually, medieval philosophers would come up with three terms: ens (an existing thing), essentia (the nature of a thing), and esse (the act or property of being/existing). So, every thing that actually exists (ens) possesses both a particular nature (essentia) and the property of existing (esse).

Well I agree with Kingshorsey about Essentia. If I talked about Unicorns, or Justice, or Beauty, you know what I mean because you know their essence: essentia.

And if I asked you to describe them you might start with 'spiral horns and white hair;' or 'wigs, black robes, and juries;' or 'angel choirs and tangerine skies.' These are
accidental properties which remind you of the 'Essentia.'

But in this world Unicorns are not alive; Justice turns out to be about plea bargaining and how much you can afford for a lawyer; and beauty means hours of sweat in a gym or sleepless nights creating a novel.

What Unicorns don't have; but what Law, and ballet, and novels have is an extra something called 'ΥΛΗ

I think the word you are looking for is 'ΥΛΗ or hyle. Accusative hylen.

But I'm just an apocalyptic platonist.