Actuality and Potentiality
An acorn is not an oak. It is also not unrelated to one, in the way a stone is. Aristotle's answer is that being comes in two modes: actuality (energeia) — how a thing is now — and potentiality (dunamis) — what it has it in itself to become, given the right conditions.[2] Actualization is the passage from the second to the first, and on this account it is what change is: not one thing replaced by another, but a potential in a subject being realised.
The distinction was invented to solve a real problem. Parmenides had argued that change is impossible, since a thing cannot come from what is (it is already there) or from what is not (nothing comes from nothing). Potentiality is the third option: the oak comes from something that was neither actually-oak nor nothing, but potentially-oak. The move is doing metaphysical work, not just naming the obvious — it says a thing's possibilities are features of the thing, as real as its current shape, which a strict materialist reading of the world has to either explain or explain away.
Aristotle's. Actual as opposed to potential, above. Related to entelecheia — being fully at work, having reached the end that defines the kind of thing you are. This sense carries a teleological load: a potential is always a potential for some determinate outcome.
The modal logician's. In contemporary metaphysics "the actual world" means this world as opposed to the merely possible ones used to analyse necessity and possibility. Actualism is the view that only actual things exist — possible worlds are ways this world could have been, not places. That is a different debate that happens to share a word.
The psychologist's. Self-actualization — Maslow's and Rogers' term for realising one's capacities — borrows Aristotle's vocabulary and belongs to a separate lineage. If you arrived here looking for that, it sits with psychology, not metaphysics.