Defining 'ontological category'

Although a considerable degree of precision has been introduced both into the formulation and the discussion of ontological theories by the use of formal methods there is still a remarkable indefiniteness about foundational issues. In particular it is not clear what an ontological category is and why we regard something as an ontological category. This is amazing given that the notion of ontological category is in fact the most basic of the whole of ontology: it is what this discipline is about.