Reality is the sum or aggregate of all that is real or existent within a system, as opposed to that which is only imaginary, nonexistent or nonactual. The term is also used to refer to the ontological status of things, indicating their existence. In physical terms, reality is the totality of a system, known and unknown.

Philosophical questions about the nature of reality or existence or being are considered under the rubric of ontology, which is a major branch of metaphysics in the Western philosophical tradition. Ontological questions also feature in diverse branches of philosophy, including the philosophy of science, of religion, of mathematics, and philosophical logic. These include questions about whether only physical objects are real (i.e., physicalism), whether reality is fundamentally immaterial (e.g. idealism), whether hypothetical unobservable entities posited by scientific theories exist, whether a 'God' exists, whether numbers and other abstract objects exist, and whether possible worlds exist. Epistemology is concerned with what can be known or inferred as likely and how, whereby in the modern world emphasis is put on reason, empirical evidence and science as sources and methods to determine or investigate reality.