A story where the hero leaves our world for another one, by dying, being summoned or falling through a portal. The word is Japanese for "another world". Many LitRPG series open this way, and some readers just say "portal fantasy".
In Bog Standard Isekai, Mark is reborn as a child in a burned-out village; in Beneath the Dragoneye Moons, Elaine "is ripped from this world to Pallos".