Ontological: 'In metaphysics, ontology is the philosophical study of being, as well as related concepts such as existence, becoming, and reality.' [LINK: https://en.wikipedia.org/wiki/Metaphysics]

The definition given is accurate and unobjectionable as stated (ontology as the philosophical study of being, existence, and reality is standard usage, correctly summarized even in the linked general reference). Where 'ontological' arguments specifically are concerned, the Kantian worry that 'existence is not a real predicate' is a serious historical objection, but it targets Anselm's original modal-ontological move more than it defeats every modern version. Alvin Plantinga's modal ontological argument (*The Nature of Necessity*, 1974) is explicitly built to sidestep the Kantian objection: it does not treat existence as a predicate added to a concept, but reasons within S5 modal logic from the mere possibility of a maximally great being (one that is omniscient, omnipotent, and morally perfect in every possible world) to its necessary actual existence, given that necessary existence is part of what 'maximal greatness' means. The argument is formally valid; its only substantive premise is that a maximally great being is possible, which is contestable but not obviously question-begging, and which most objectors, including many atheist philosophers, concede is the argument's actual weak point rather than a Kantian predicate confusion.

Classical theism's ontology of God as ipsum esse subsistens (Aquinas) — not a being among beings but Being itself, the ground of all contingent existence — gives content to why God, if he exists, would exist necessarily and be the metaphysical foundation of everything else that merely happens to exist, tying the ontological argument's conclusion to a broader coherent doctrine of God rather than an isolated logical trick.