Descartes uses the term in his third meditation (Med. III) to demonstrate the existence of God, see a previous question.
The term “realitas objectiva” is a technical term from scholastic ontology.
How is the term defined, how does it apply, which explanatory power does the concept have?
Can you give a reference?
Why God is considered the “ens realissimum”?
I would appreciate any help from experts in scholastic theology.
Heidegger describes an inversion of meaning of realitas objectiva from Descartes and Scholasticism to Kant and modernism (depending on your subscription), in The Basic Problems of Phenomenology, Chapter One Kant's Thesis: Being Is Not A Real Predicate p. 37–38.
[Kant] separates objective reality as actuality from possibility. If I devise or invent some possible thing, then in doing so I am occupied with this imagined thing's pure relationships having real content, though without thinking of the thing with these relations as being actual, presently existent. In retrospect, this use of reality occurs also in Descartes. Descartes says, for instance, that error, and in general everything that has negative value, everything malum, non esse quid reale, is nothing.11 This does not mean that error does not actually exist; instead, error is surely actual, but it and everything evil and bad is not a res in the sense that it would be an independent real content for itself. It is always only advenient and it is only by means of the negation of an independent real content, by the negation of the good. Similarly in the proof for God's existence in the third meditation, when he is speaking of realitas objectiva and realitas actualis, Descartes here, too, takes realitas in the sense mentioned above—the sense of realness or res-ness, German Sachheit—equivalent to the Scholastic quidditas [whatness, somethingness]. Realitas objectiva is not identical with the Kantian objective reality but just the opposite. In Descartes realitas objectiva means, following Scholasticism, the objectified what, which is held over against me only in pure representation, the essence of a thing. Realitas objectiva equals possibility, possibilitas. In contrast, what corresponds to the Kantian concept of objective reality, or actuality, is the Cartesian and Scholastic concept of realitas actualis—the what which is actualized (actu). This noteworthy distinction between the Cartesian concept of realitas objectiva as tantamount to subjectively represented possibility and the Kantian concept of objective reality, or that which is in itself, is connected with the fact that the concept of the objective [Objektive] was turned into its exact opposite during this period. The objective, namely, that which is merely held over against me, is in Kantian and modern language the subjective. What Kant calls the subjective is for the Scholastics that which lies at the basis, hupokeimenon, the objective, thus corresponding to the literal sense of the expression "subject."
- Descartes, Meditationes de prima philosophia, Latin-German edition (Felix Meiner, 1959). Meditation 4, p. 100.
Ens realissimum is mentioned earlier on page 37 and on page 148.
