Existence, in its most generic terms, comprises the state of being real[1] and the ability to physically interact with the universe or multiverse. What existence is exactly is up for interpretation; and, is one of the most important and fundamental topics of ontology, the philosophical study of the nature of being, existence, or reality in general, as well as of the basic categories of being and their relations. Traditionally listed as a part of the major branch of philosophy known as metaphysics, ontology deals with questions concerning what entities exist or can be said to exist, (for instance: "Does the stellar structure UDFj-39546284 exist?"), and how such entities can be grouped, related within a hierarchy, and subdivided according to similarities and differences.

Materialism holds that the only things that exist are matter and energy, that all things are composed of material, that all actions require energy, and that all phenomena (including consciousness) are the result of material interactions.

Life is a characteristic which distinguishes objects that have self-sustaining biological processes from those that do not[2][3]—either because such functions have ceased (death), or else because they lack such functions and are classified as "inanimate".[4]

In mathematics, existence is asserted by a quantifier, the existential quantifier, one of two quantifiers (the other being the universal quantifier). The properties of the existential quantifier are established by axioms.