In Star Control II, a dimension (sometimes called universe1) is one of various, distinct "realms of existence," each of which appears to be a self-contained, expansive continuum of space-time with its own set of values for certain basic physical constants. Some of these regions map on to each other and appear to exist within the same "space" but with differing "reality phases". However, the differences in reality phase prevent most interactions between objects in different dimensions, making each dimension effectively invisible to other dimensions. Thus, objects cannot travel between them without the use of special technology.
Terminology and Classification
Though similar phenomena had been speculated about as "branes" by Human physicists in the 21st century, dimensions other than TrueSpace were not known by Humans to exist until their discovery of HyperSpace, which was thought by Humans and most other Alliance races to be unique until recently. The English term "dimension" in this context was first coined by Androsynth scientists, by analogy with similarly named entities in Human speculative fiction, named as a shorthand for what are properly multiple sets of classical dimensions separate from the four TrueSpace dimensions we habitually move through. The Androsynth and the Precursors were the only TrueSpace races known to have done serious work studying dimensional physics (it is speculated that the Taalo may have also). The Arilou intimate that, although the trivialized concept of "dimension" is a suitable metaphor for the Humans' intellect, the actual way these other realms of existence exist and interact with each other is much more complex; the Arilou are not from our "space" or "*time*". The Orz, likewise, explain that "*Time* is not one but many. *Space* is many. *Colors* are many. You are so *sticky*. You cannot *slide* like Orz from *outside* to *inside* and *in between*".
Before the discovery of the Androsynth discipline of dimensional physics, a common metaphor for HyperSpace's relationship to TrueSpace was that of two planes parallel to each other in space, one "above" the other. This gives rise to the metaphor of "pushing up" into HyperSpace (itself a term meaning "above space") and "falling down" into TrueSpace, and the metaphor of dimensions as "higher dimensions" to TrueSpace. The Pkunk and some others also commonly use the term "plane" for what others call dimensions.