I have been using the English “actuality”, following old standard translations of Aristotle. As with any Aristotelian technical term, in interpreting its meaning I try to rely on what the Aristotelian texts say about it, and to avoid importing connotations associated with other uses of the English word used to translate it. Aristotle’s Greek term is energeia, a word he apparently invented himself from existing Greek roots. Joe Sachs translates it as “being at work”, which I think is good provided “being” is taken in the ordinary sense that we transitively say something “is” at work, rather than taking “being” as a noun. The word is formed from the noun ergon, which in its root sense means “work”; the prefix en, which corresponds to the preposition “in”; and the suffix eia, which makes the whole thing into a noun, like English “-ness” or “-ity”. So, in the most literal sense, energeia means something like “in-work-ness”.
Even the literal sense is a bit misleading, because Aristotle is very clear that the primary reference of energeia is not to a present state or a factual state of affairs, but to a primitive or ultimate end, understood as a kind of fullness or achievable perfection after its kind.
We are not used to thinking seriously about achievable perfection, but Aristotle’s fundamental intention regarding “perfection” is that it not be out of reach of finite beings. The “perfection” Aristotle has in mind is not a godlike attribute of unqualified or infinite perfection, but rather something like what modern ecology calls a “climax state” of an ecosystem (like the exceedingly rich environment of a rain forest).
Ecological succession involves a series of states that lead to other states, whereas a climax state leads back to itself, as in Aristotle’s other related coined word entelechy, which Sachs renders “being at work staying itself”, and is literally something like “in-end-having”. An ecosystem in a climax state is maximally resilient to perturbation; it is more able to recover its health when something throws it out of balance.
When Aristotle speaks of “substances” persisting through change, it is not a simple persistence of given properties that he has in mind, but rather something more like a stable (i.e., highly resilient, not unchanging) ecosystem. Stability in ecosystems and populations comes from biodiversity, which is a modern scientific analogue of Aristotelian “perfection”. Diversity provides a richer set of capabilities. With respect to human individuals, the analogue would be something like a “well-rounded” character. In ethics, we could speak of a well-rounded pursuit of ends, in contrast with a narrow or selfish one.
Thus the concept of “actuality” in Aristotle has to do with a kind of immanent teleology or interpretation of things based on ends and values, which for Aristotle takes the place of what later writers called “ontology”, as a supposed fundamental account of what exists.
Some contemporary analytic philosophers have spoken of “actualism” as an alternative to the possible worlds interpretation of modal logic. According to the Stanford Encyclopedia of Philosophy, in this context “actuality” is simply equated with factual existence. Instead of making confusing claims about the reality of non-actual possible worlds, this approach locates alternate possibilities within the actual world instead of somehow alongside it. As far as it goes, I have some sympathy for this. But I want to resist some of the conclusions with which it is commonly associated, which follow from the very non-Aristotelian identification of actuality with mere factuality. (See also Redding on Morals and Modality.)
I said above that actuality in Aristotle’s sense refers to processes and states of actualization relative to ends and values, not just to present existence or the current factual state of affairs. Readers of Aristotle as diverse as Thomas Aquinas and Gwenaëlle Aubry are united in stressing the primacy of “in-act-ness” over mere factuality in the interpretation of “actuality”. Robert Pippin’s account of actuality in Hegel (see also here) in an ethical context spells out the consequences of this very nicely. I think Aristotle would endorse the views Pippin attributes to Hegel in this context.