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The **three-dimensional torus**, or **3-torus**, is defined as any topological space that is homeomorphic to the Cartesian product of three circles, In contrast, the usual torus is the Cartesian product of only two circles.

The 3-torus is a three-dimensional compact manifold with no boundary. It can be obtained by "gluing" the three pairs of opposite faces of a cube, where being "glued" can be intuitively understood to mean that when a particle moving in the interior of the cube reaches a point on a face, it goes through it and appears to come forth from the corresponding point on the opposite face, producing periodic boundary conditions. Gluing only one pair of opposite faces produces a solid torus while gluing two of these pairs produces the solid space between two nested tori.

In 1984, Alexei Starobinsky and Yakov Zeldovich at the Landau Institute in Moscow proposed a cosmological model where the shape of the universe is a 3-torus.^{[1]}

**^**Overbeye, Dennis.*New York Times*11 March 2003: Web. 16 January 2011. “Universe as Doughnut: New Data, New Debate”

- Thurston, William P. (1997),
*Three-dimensional Geometry and Topology, Volume 1*, Princeton University Press, p. 31, ISBN 9780691083049. - Weeks, Jeffrey R. (2001),
*The Shape of Space*(2nd ed.), CRC Press, p. 13, ISBN 9780824748371.