Actus Essendi is a Latin expression coined by Thomas Aquinas. Translated as 'act of being,' the expression actus essendi refers to a fundamental metaphysical principle discovered by Saint Thomas Aquinas (1225–1274) in his Christianizing of Aristotle.
Aquinas saw that in any subsisting extramental thing one finds a couplet of metaphysical principles: one is the ‘essence’ which makes the thing to be what it is, the other is the actus essendi which gives to the thing and to its ‘essence’ actual existence.
The observation that individual things display instantiations of a particular ‘essence’ led Aquinas to postulate that what gives actual existence to a thing and to its ‘essence’—the actus essendi—is unique, in the sense that the perfection of actus essendi cannot be said to be common in the way an ‘essence’ is said to be common.
Things instantiating the essence of horseness, for example, are said to be similar because of their horseness. The essence of horseness is what makes horses to be the same under a common category.
But things instantiating the perfection of actus essendi, are said to be different on account of their actus essendi. The possession of actus essendi is what makes things unique and distinct from all other things.
Thus in what actually exists as a subsisting extramental thing, there is an ‘essence’ which makes the thing to be what it is (a horse, for example), and the actus essendi which makes the thing to be a real, individual, existing thing.
Aristotle didn’t have the notion of actus essendi. In fact, the contribution of Aquinas to the philosophy of being is precisely this, that he discovered that all Aristotelian acts were in reality ‘potency’ with respect to the actus essendi.
Aquinas saw that the metaphysical principle of actus essendi is the “act of all acts, the perfection of all perfections,” and “a proper effect of God.” The metaphysics of Aristotle did not reach that far.
Accordingly, Pope John Paul II stresses in his teachings that the philosophy of Saint Thomas Aquinas is the philosophy of actus essendi, "whose transcendental value paves the most direct way to rise to the knowledge of subsisting Being and pure Act, namely to God." Aquinas defined God as the ‘Ipse Actus Essendi subsistens,’ subsisting act of being.