Ontology

Ontology is the philosophical study of the nature of "being, "becoming", existence", or "reality". It is part of the major branch of philosophy known as metaphysics. 
Ontology deals with questions about what things exist or can be said to exist, and how such entities can be grouped according to similarities and differences. 
Overview.
Ontology asks whether "categories of being" are fundamental. 
Some philosophers, of the Platonic school, say that all nouns (including abstract nouns) refer to actual entities. Other philosophers contend that nouns do not always name entities. They think some are a kind of shorthand for a collection of either objects or events. 
In this view, "mind", instead of referring to an entity, refers to a collection of "mental events" experienced by a "person". "Society" refers to a collection of "persons" with some shared characteristics, and "geometry" refers to a collection of a specific kind of intellectual activity. 
Between these poles, called realism and nominalism, are other positions. Any ontology must give an account of which words refer to entities, which do not, why, and what categories result.