Aparallel world, parallel universe, or alternative reality is a self-contained separate reality coexisting with our own. Because "alternative reality" sometimes implies that the reality is a variant of our own, like for example a mirror world, we use the term "parallel world/universe" which is more general, without any connotations implying a relationship (or lack thereof) with our own world/universe (meaning a fictional world can just as well have an alternative reality, it doesn't need to be our reality). To put it more simply, this category is to show if more than 1 world/universe/reality is present in the anime.

Multiple timelines of the same world/universe/reality areNOT considered parallel worlds. For example, inSteins;Gate the cast travels through multiple "World Lines", which are defined as parallel timelines of thesame universe had various events occurred with differing outcomes.