The concept that there is another dimension beyond the three that we use to describe or visualize solid objects or describe mathematically in solid geometry. The idea entered the popular realm when explanations of Einsteins special theory of relativity appeared in the media. Einsteins theory made time the fourth dimension, inextricably connected with the three spatial dimensions, constituting the four-dimensional space-time continuum. The implications of this were negligible for everyday life but of great importance for some elements of physics and cosmology. To further complicate things, mathematicians and theoretical physicists had no difficulty in using four-dimensional mathematics and formulating five-, six-, seven- or more dimensional geometries (a good explanation of this for the layman is in Antony G. N. FLEW’s A New Approach to Psychical Research, Appendix If). The fourth dimension is time, which in our everyday existence is independent of the three space dimensions; the interconnectedness of the three dimensions of space and the fourth dimension of time does not matter. To a particle traveling at about the speed of light (186,000 miles a second), the connection between space and time does matter. There isn’t any scientifically accepted evidence for another dimension.
The multidimensional idea was seized upon by some exploring the supernatural, notably by John William Dunne, to explain spiritualism, and pre- and retrocognition.