Ontology

From the Quicksilver Metaweb.

Ontology is the (philosophical) study of what is. That is to say, it deals with questions of what things really exist in and of themselves - a very simple example is whether numbers actually exist, or are just... something else. This latter question addresses part of the ontology of (Philosophy of) Mathematics - another use of the word is as the noun for the accepted or stipulated "objects" of a subject.

Ontology is often confused with metaphysics, which the the study of the nature of what exists. Both tie closely with the question of substance, an attributed posited to things that exist.