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In the same way that the Moon refers to our (Earth's) moon, the Universe is used by some cosmologists to refer to our Universe. In this article, the Universe is equivalent to our observable Universe.

Theoretical and observational cosmologists vary in their usage of the term the Universe to mean either this whole system or just a part of this system.^[2]

As used by observational cosmologists, the Universe most frequently refers to the finite part of space-time. The Universe is directly observable by making observations using telescopes and other detectors, and by using the methods of theoretical and empirical physics for studying its components. Physical cosmologists assume that the observable part of (comoving) space (also called our universe) corresponds to a part of a model of the whole of space, and usually not to the whole space. They use the term the Universe ambiguously to mean either the observable part of space, the observable part of space-time, or the entire space-time.^{[citation needed]}

In order to clarify terminology, George Ellis, U. Kirchner and W.R. Stoeger recommend using the term the Universe for the theoretical model of all of the connected space-time in which we live, universe domain for the observable universe or a similar part of the same space-time, universe for a general space-time (either our own Universe or another one disconnected from our own), multiverse for a set of disconnected space-times, and multi-domain universe to refer to a model of the whole of a single connected space-time in the sense of chaotic inflation models.^{"PHUC QUANG PHAM -UNIVERSYTY OF TECHNOLOGY DA NANG -INFORMATION"}