Dimensional fatigue — also DF, Inter-Dimensional Fatigue, or IDF — is a catchall term for processes or forces that weaken the boundaries between "dimensions" or space-times, allowing information or objects to pass from one to the other. The form of DF most familiar to the races of the Alliance is the QuasiSpace Portal Spawner, used to access QuasiSpace from HyperSpace.
The only race known to be familiar with DF is the Arilou Lalee'lay, who provided the Portal Spawner for Alliance use during the war but otherwise strongly discourages DF research as incredibly dangerous. The last known race to have attempted such research, the Androsynth, were utterly destroyed as a result. What records exist seem to indicate that acquiring knowledge such as images and other information through experimental DF technology somehow made the Androsynth visible to entities that exist in other dimensions. The Androsynth's research led to a series of events such as anomalous regions of high gravity ("mosquito mange"), spontaneously generated forces ("poltergeists"), sensory manifestations ("ghosts"), and other "reality aberrations," which may have been spontaneous DF events generated by those entities on the other side.1 In the end, this is probably somehow responsible for the race's disappearance and replacement by the trans-dimensional Orz. The Arilou claim to have protected at least the race of Humans from a similar fate by unknown measures, but warn that any increased knowledge among Humans of DF phenomena would create similar vulnerabilities.
There is at least one naturally occurring DF phenomenon, the bi-directional QuasiSpace portal near the Circini and Chandrasekhar star clusters. Interdimensional fatigue is also responsible for the disappearance of the star Alpha Corvi and its planetary system, though whether this was a natural event or not is unknown.2