Ontology.

In philosophy, ontology is the study of being or existence and forms the basic subject matter of metaphysics. It seeks to describe or posit the basic categories and relationships of being or existence to define entities and types of entities within its framework.

Ontology can be said to study conceptions of reality; and, for the sake of distinction, at least to the extent to which its counterpart, epistemology can be represented as being a search for answers to the questions "What do you know?" and "How do you know it?", ontology can be represented as a search for an answer to the question "What are the knowable things?".

Some philosophers, notably of the Platonic school, contend that all nouns refer to entities. Other philosophers contend that some nouns do not name entities but provide a kind of shorthand way of referring to a collection (of either objects or events). In this latter 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 interactions, and geometry refers to a collection of a specific kind of intellectual activity.

Any ontology must give an account of which words refer to entities, which do not, why, and what categories result. When one applies this process to nouns such as electrons, energy, contract, happiness, time, truth, causality, and God, ontology becomes fundamental to many branches of philosophy.

Ontology has one basic question: "What is there?" Different philosophers provide different answers to this question.

One common approach is to divide the extant entities into groups called "categories". However, these lists of categories are also quite different from one another. It is in this latter sense that ontology is applied to such fields as theology, information science and artificial intelligence.
Further examples of ontological questions include:

What is existence?

Is existence a property?

What does it mean to say something does not exist?

Are sentences expressing the existence or non-existence of something properly called propositions?

What is a physical object? Can one give an account of what it means to say that a physical object exists?

What constitutes the identity of an object?

When does an object go out of existence, as opposed to merely changing?

What features are the essential, as opposed to merely accidental, attributes of a given object?

What are an object's properties or relations and how are they related to the object itself?

What could it mean to say that non-physical objects (such as a time, souls) exist?

Why are we here? Why does anything exist, rather than nothingness? (Though, according to some, these questions may be more in the realm of cosmology.)