The Blue Marble, a photograph of the planet Earth made on 7 December 1972 by the crew of the Apollo 17 spacecraft.

In its most general sense, the term "world" refers to the totality of entities, to the whole of reality or to everything that was, is and will be. The nature of the world has been conceptualized differently in different fields. Some conceptions see the world as unique while others talk of a "plurality of worlds". Some treat the world as one simple object while others analyze the world as a complex made up of many parts. In scientific cosmology the world or universe is commonly defined as "[t]he totality of all space and time; all that is, has been, and will be". Theories of modality, on the other hand, talk of possible worlds as complete and consistent ways how things could have been. Phenomenology, starting from the horizon of co-given objects present in the periphery of every experience, defines the world as the biggest horizon or the "horizon of all horizons". In philosophy of mind, the world is commonly contrasted with the mind as that which is represented by the mind. Theology conceptualizes the world in relation to God, for example, as God's creation, as identical to God or as the two being interdependent. In religions, there is often a tendency to downgrade the material or sensory world in favor of a spiritual world to be sought through religious practice. A comprehensive representation of the world and our place in it, as is commonly found in religions, is known as a worldview. Cosmogony is the field that studies the origin or creation of the world while eschatology refers to the science or doctrine of the last things or of the end of the world.

In various contexts, the term "world" takes a more restricted meaning associated, for example, with the Earth and all life on it, with humanity as a whole or with an international or intercontinental scope. In this sense, world history refers to the history of humanity as a whole or world politics is the discipline of political science studying issues that transcend nations and continents. Other examples include terms such as "world religion", "world language", "world government", "world war", "world population", "world economy" or "world championship".