A dimension is sort of an infinite plane of existence within a universe. Several can exist in the same place at the same time because they vibrate differently. The laws of physics do not always follow the same patterns, depending on dimension, and crossing between them is hazardous unless you do it just right. Anyone who's been to the higher dimensions knows they're a pretty nasty heathen lot up there who should just be smashed and done in, and would be, too, if anyone could work out a way of firing missiles at right angles to reality.
Dimensions are grouped into collections called Pandimensionalities. Grouping criteria are completely arbitrary and vary depending on which scientist or database you consult. We're doing our part to add to the confusion by not listing any.
Dimension Q is the largest dimension by far, encompassing several universes in contrast to the norm. Hence everything in this encyclopedia is from this dimension unless otherwise noted.