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*, *space* and *colors* are many, and that, because Humans are *sticky*, they cannot *slide*. Some of the Orz best-fits seem to be related to dimensions and the description of dimensions, such as *level*, *space*, *time*, *colors*; other best-fits seemingly related to dimensions refer to things one can be "in" or "from" — *above*, *the middle*, *below*, *outside*, *in between*, *inside*, *heavy space*, *Pretty Space*, *slow time*, and *quick time*; other best-fits are apparently related to the concept of moving between dimensions — *pull*, *slide*, *spread*, and *HYUIVBHJHG*.
Prior to the current knowledge of additional dimensions, when HyperSpace was still the only known parallel dimension, a common metaphor for HyperSpace's relationship to TrueSpace was that of two planes parallel to each other, one "above" the other (HyperSpace itself meaning "above space"). This metaphor gives rise to the phrases used to describe interdimensional travel, such as "pushing up" to refer to the translation from TrueSpace into HyperSpace and "spiralling down", "warp[ing] down", or "drop[ping] down" when going from HyperSpace to TrueSpace.2 While this metaphor does not coincide perfectly with the concept of reality phases, it does seem to match up with some of the best-fits the Orz use when discussing the concept of dimensions — specifically, they seem to refer to QuasiSpace as "*above*" and the Orz home dimension as "*below*".