Ontological+Category

In [|metaphysics] (in particular, [|ontology]), the different kinds or ways of [|being] are called **categories of being** or simply //categories.// To investigate the categories of being is to determine the most fundamental and the broadest [|classes] of entities. A distinction between such categories, in making the categories or applying them, is called an **ontological distinction**. (wikipedia)

Or

the branch of metaphysics that studies the nature of existence or being as such. (Dictionary.com)