When it comes to describing a fictional world or setting, there are a number of synonyms that can be used in place of the term "fictional universe". Some possible alternatives include "imaginary realm", "fictional realm", "fantasy world", "fantasy realm", "make-believe universe", "created world", "created realm", "invented world", "invented realm", "illusory universe", "imagined world", and "imagined realm". Depending on the context and tone of the writing, different synonyms may be more appropriate than others. However, all of these terms convey the idea of a fictional or imaginary setting that is distinct from the real world.