(Latin: ex, out of; stare stand)
Is that by which a thing is actually costituted in the physical order, ie., in the order of actuality as distinct from the order of mere possibility.
It corresponds to the concept of being taken in the participial sense of the term.
Existence has been called the actuality of essences, namely that by which a being has its place in the order of actuality, as distinet from the metaphysical order or order of essences.
So simple a concept does not admit of strict definition or even of explanation in term simpler than itself.
We arrive at the idea of existence first from our own internal experience by which we perceive ourselves and our own subjective states to be actualities.
The etymological derivation of the term is illuminating provided we restrict the application of it in this root sense to the order of finite, produced reality.
For, an efficient course or agent, before it actually produces an effect, virtually contains that effect, ie., has the power to produce the effect not yet existing; but when it has exercised that power, then the effect produced is rightly said to stand (or be constituted) outside its cause.
New Catholic Dictionary