The study of “being as being”, independent of its particular determinations. In modern philosophy, a distinction has been made between the study of being as essence: the causes and laws of being (essentialism), and the study of being as existence: concrete, singular beings, the beings (existentialism).
Since being is beyond its particular determinations, onto (being)-logy (science) is not so different from meta (beyond)-physics (beings). Nevertheless, Kantian and Heideggerian critics have shown that ontology – like metaphysics – sometimes degenerates into a (closed) system, losing its openness to being (Wesen) and being reduced to the study of being (Dasein), with God himself being reduced to the status of prime Being (ontotheology).
To avoid this problem of vocabulary variation, we prefer to speak of metaphysics whenever the study concerns the beyond of being rather than being, or being rather than beings.