The Outside is that which lies beyond the World.
Most commentators follow Ajencis's so-called Dyadic Theory when characterizing the World and its relation to the Outside. In Meta-Analytics, Ajencis argues that it is the relation between subject and object, desire and reality, that underwrites the structure of existence. The World, he argues, is simply the point of maximal objectivity, the plane where the desires of individual souls are helpless before circumstance (because it is fixed by the desire of the God of Gods). The many regions of the Outside then represent diminishing levels of objectivity, where circumstances yield more and more to desire. This, he claims, is what defines the "spheres of dominance" of Gods and demons. As he writes, "the greater will commmands." The more powerful enitities of the outside dwell in "sub-realities" that conform to their desires. This is what makes piety and devotion so important: the more favour an individual can secure in the Outside (primarily through the worship of gods and the honouring of ancestors), the greater the chance of finding bliss in the afterlife.