Perfection, completion, fulfillment, or realization. Although in many cases it is the terminus of a movement or actualization of a potency of some kind, it need not be. When it is not, perhaps it is best described as perfection rather than completion or fulfillment since the latter two connote a movement to a terminus whereas perfection does not necessarily bear this connotation. 

Entitative Act.— The act by which a thing exists; existence considered in relation to the subject of existence.   

Operative Act (also called Operation).— An action done by a thing that exists. E.g., thinking, talking, or running.   

Pure Act.— Act unlimited in any way by potency. Act of this kind can only be attributed to God. 

First Act (also called Primary Act).— A fundamental act of a thing in some order or other. E.g., in the order of being, existence (entitative act) is the first act of a thing whereas in the order of operation, a power will be the first act, as, e.g., intellect is first act with respect to its various operations. 

Second Act (also called Secondary Act).— An act in any order that presupposes a first act.