Metalepsis

Genette defines narrative metalepsis as “any intrusion by the extradiegetic narrator or narratee into the diegetic universe (or by diegetic characters into a metadiegetic universe, etc.) or the inverse” (234–35). Metalepsis thus designates the transgression of a line of demarcation that authors usually do not touch, namely the “shifting but sacred frontier between two worlds, the world in which one tells, the world of which one tells” (236).

To put it simply, it occures when we move between 2 different worlds in a same text, when we find more than one fictional world.

Genette, Gérard ([1972] 1980). Narrative Discourse: An Essay in Method. Ithaca: Cornell UP.