Three-dimensional space (also: tri-dimensional space or 3-space) is a geometric three-parameter model of the physical universe (without considering time) in which all known matter exists. These three dimensions can be labeled by a combination of three chosen from the terms length, width, height, depth, and breadth. Any three directions can be chosen, provided that they do not all lie in the same plane.

Trailokya (; , ) has been translated as "three worlds," "three spheres," "three planes of existence," "three realms" and "three regions." These three worlds are identified in Hinduism and already appear in early Buddhist texts.