Actus means determination, complement, perfection. In every being there are many actualities, and these are subordinated. The complete substance constituted by these two principles receives further deternimations, which are, in that respect, second actualities. Yet these may also be conceived as first actualities. Thus the extensive quantity of a substance is a first actuality when compared to the shape.
Power is a first actuality when compared to action.
And this is the most frequent application of the terms actus primus and actus secundus . Remy Lafort, S.T.D., Censor Imprimatur.