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Isekai

Isekai (Japanese: 異世界, transl. "different world" or "otherworld") is a genre of light novels, manga, anime and video games that revolve around a normal person from Earth being displaced, transported to, reborn or otherwise trapped in a parallel universe, fantasy world, or virtual world, where they must adapt and/or assimilate into this new world, along with its new rules, cultures and philosophies. By taking a person from the "real world" readers know, and somehow transport them into the fantasy world, this allows them to learn about the new world at the same pace as the protagonist. This requires more agency on the part of the audience, and makes it much easier for readers to be invested unto these worlds. 

Isekai can include fantasy or science fiction elements, and the term typically refers to Japanese fictional works. It has similarities to accidental travel and portal fantasy stories in young adult literature, prior to Japanese writers eventually coining a single word "Isekai" to sum up these types of stories.

In some versions, the protagonist is depicted as being already familiar with the parallel world, as it is often a fictional universe from a fictitious work published in the protagonist's origin universe (see Sonic Storybook Series). It is equally likely the parallel world may also be unknown to them, as is the case with 'Reverse Isekai' stories such as Sonic X, No Game No Life, and Isekai Quartet. The new universe can be an entirely different alien world where only the protagonist retains knowledge and has any memory of their former life, as in Saga of Tanya the Evil and Ascendance of a Bookworm. 

How the protagonist gets there can vary greatly: in some media, they are reincarnated into that world after dying; in others, they are summoned or teleported to that world by accidental or intended reasons, as in the case of Isekai Cheat Magician; or they may unknowingly have walked through a portal connecting two worlds, as in the case of Spirited Away and Inuyasha. In Sorcerous Stabber Orphen, an entire population of humans appeared in the magically created world was transported from Earth and got partially mixed with local dragonlike Heavenly Beings. Isekai may also take place in the virtual world of a massively multiplayer online (MMO) game, such as .hack and Sword Art Online, and it may also be one where a formerly virtual world turns into a real one, such as in Log Horizon and Overlord.