Aristotelian energeia — traditionally translated as actuality — is a nominalized adverbial expressive metaconcept capturing the status of being active or effectively operative in a process. I have somewhat awkwardly rendered it as “at-work-ness”. “Being-at-work” sounds like better English, but might wrongly be taken to refer to a kind of Being in the intransitive sense qualified by a predicate of at-work-ness. (I think Aristotle was in fact very little interested in Being in an intransitive sense. He devotes much more attention to several transitive senses.) There is no “being” at all in the Greek. Energeia is most literally “in-work-ness”, but I and others have preferred to substitute “at” for “in”, as better conveying the intended connotation in English.
Contrary to Plato’s doubts about the possibility of understanding becoming, Aristotle is committed to eliciting its intelligibility. Rather than looking for generative powers behind things as Plato had obscurely suggested might be our best hope, part of Aristotle’s strategy is to draw our attention to what is immanently at work in a process as a kind of methodological starting point. The discernment of what it was to have been such and such a thing begins from the indistinct apprehension of something we merely take to have been effectively operative. (That something would be a mediated immediacy in Hegelian terms.) It is eventually constituted with greater precision and a degree of universality through inferential elaboration of the counterfactual potentiality of what we initially took to have been effectively operative, as well as through the implicit correction over time of errors that become apparent in the course of this elaboration.
Worlds away from the dry stereotype of “essentialism”, Aristotle is if anything more of a process thinker or pragmatist. He directs our attention to the concrete actualization of things, which “essentially” involves the interweaving of effectively operative actuality with both counterfactual potentiality and material contingency. Hegel makes large use of this Aristotelian concept. Brandom associates Hegelian actualization with expression and making explicit.
There is a very interesting distinction suggested by Aristotle and developed by later writers between a “first” and “second” actuality. Whereas the first actuality of an organic body is not too far from the later Stoic conatus as an internal source of primitive desiring activity, second actuality applies to things associated with evolved practice like habit, character, and intellect.
Aristotle also speaks about the “First” cause as pure at-work-ness, with no admixture of potentiality. I take this to mean that the “First” cause — just as the higher-order goal at which everything indirectly aims — is effectively operative in things, but unlike other effectively operative things, it has no counterfactual aspect (because it has no factual aspect, because it exactly is a pure aim rather than something having an aim). It functions as an ideal of normativity that we can retroactively see to have been at work, as a sort of virtual, uplifting attractor of purely natural desire, and also more speculatively as a posited virtual attractor for the directionality in material tendencies. (See also Moved, Unmoved.)