Ontology:
1) a branch of metaphysics concerned with the nature and
relations of being, 2) a particular theory about the nature of
being or the kinds of existents.
Ontology and Metaphysics are very similar. The dictionary defines the latter as "a division of philosophy that is concerned with the fundamental nature of reality and being." Sounds almost the same as ontology, doesn't it!?
Another definition of Metaphysics that I find interesting was used by Immanuel Kant in the introduction to his incomplete and posthumously published essay regarding What Real Progress Has Metaphyics Made in Germany since the Time of Lebniz and Wolff? It goes like this:
"Metaphysics is the science of advancing by reason from knowledge of the sensible to knowledge of the supersensible."
In that same work, he says, "Ontology (as a part of metaphysics) is the science that comprises a system of all concepts and principles of understanding, but only insofar as these extend to objects given by the senses and can, therefore, by justified by experience. It does not deal with the supersensible, the ultimate end of metaphysics, and thus belongs to the latter only as a propadeutic. Ontology is the porch or entry way of metaphysics proper and will be called transcendental philosophy because it contains the conditions and elements of our a priori knowledge."
For even more about ontology and metaphysics, click here!