Act is undefinable. Understood by its less intelligible opposites, it is what stands to potency as its fulness and that whose loss is always in some way evil. Still, act is understood prior to potency or evil, in fact its precisely what is understood in them.
Acts are first known to us as motions, but motions are imperfect realizations of the immobile.
Immobile means neither in motion nor at the term of its motion. Immobile and restless.
Just Thomism 