Entelechy

Entelechy (entelecheia) is one of Aristotle's coinages, naming the state of a thing that has fully realised its nature and reached its end — being-at-its-completion, as against mere potential. It is closely bound to actuality: where potential is the acorn, entelechy is the full-grown oak, the form achieved and active.

Most famously, Aristotle defines the soul in On the Soul as the 'first entelechy of a natural body that has life potentially' — that is, the soul is the actuality, the realised form and functioning, of the living body, not a separate thing imprisoned within it.