Actus primus, or first actuality, is a technical expression used in scholastic philosophy.
[1] The Latin word actus means determination or complement.
The complete substance constituted by these two principles receives further determinations, which are, in that respect, second actualities.
Thus the extensive quantity of a substance is a first actuality when compared to the shape.
The modern distinction of potential and kinetic energy might serve as another illustration: the loaded gun, or the engine with steam buildup, represent first actualities; the bullet speeding to the mark, the engine flying over the rails, represent second actualities.