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Isekai (Japanese: 異世界, transl. "different world") is the portal fantasy subgenre of Japanese light novels, manga, anime, and video games. Isekai works revolve around a normal person from Earth being transported to, reborn, or trapped in a parallel universe, usually a fantasy world. Often, the protagonist is already familiar with the parallel world, as it is often a fictional universe from a fictitious work published in the protagonist's origin universe, but the parallel world may also be unknown to them, as is the case with Sonic X. The new universe can be an entirely different world where only the protagonist has any memory of their former life, as in Saga of Tanya the Evil, or one that they reincarnate in. It may also be one where a formerly virtual world turns into a real one, such as in Log Horizon and Overlord.
The subgenre can be divided into two types "transition into another world" (異世界転移 isekai ten’i) and "reincarnation into another world" (異世界転生 isekai tensei). The former, where the protagonist gets transported to another world (e.g. by traveling into it, being summoned into it, or taking possession of another being), was more common in earlier works. Whereas the latter, where the protagonist dies in their original world and is then reborn in another world, became more common in newer works.
In some works the person being transported is a NEET, shut-in, or gamer (as in No Game No Life), who in the new fantasy world, are now are able to succeed through use of their comparatively unimportant-in-real-life genre knowledge or skills, or they may have special skills or equipment, such as a game interface only they can access. Their power can range from tremendous magical abilities surpassing anyone else, as in In Another World with My Smartphone, to relatively weak, as in Re:Zero, where the protagonist does not gain any special power beyond the ability to survive death in a type of temporal loop.
While the protagonist of an isekai work is usually a "chosen hero", there have been a large number of alternative takes on the concept. In Drifters, the people entering the fantasy world are historical generals and other warriors who are more brutal than the inhabitants of the world themselves, and in That Time I Got Reincarnated as a Slime, where the protagonist starts as a slime with special abilities rather than a human. Some stories involve people being reincarnated as unusual inanimate objects, like a magical onsen. Others, known as "reverse isekai", follow beings from a fantasy universe who have been transported to or reincarnated on modern-day Earth, including the anime Laidbackers.
The concept has origins in ancient Japanese literature, particularly the story of Urashima Tarō, a widely known folk tale in Japan that isekai writers grew up with. It is about the fisherman Urashima Tarō, who saves a turtle and is brought to a wondrous undersea kingdom, but the story has a twist: after spending what he believed to be four to five days there, he returns to his home village only to find himself 300 years in the future. The folk tale was adapted into one of the earliest anime films, Seitaro Kitayama's Urashima Tarō, in 1918. Other precursors to isekai include portal fantasy stories from American and English literature, including the novels Alice's Adventures in Wonderland (1865), The Wonderful Wizard of Oz (1900), Peter Pan (1902), The Chronicles of Narnia (1950), The Phantom Tollbooth (1961), and Philip José Farmer's Riverworld series (1971).
Early anime and manga titles that could be classified as isekai include Aura Battler Dunbine (1983 debut), Fushigi Yûgi (1992 debut) and El-Hazard (1995 debut), in which the protagonists stayed similar to their original appearance upon entering a different world. Other 1990s titles identified as isekai include the novel and anime series The Twelve Kingdoms (1992 debut) and Magic Knight Rayearth (1993 debut). The anime film Spirited Away (2002) was the first world-wide known isekai anime film, although the term "isekai" was not commonly used at the time.
The .hack franchise (2002 debut) was one of the first to present the concept of isekai as an actual virtual world, with Sword Art Online (also 2002 debut) following in its footsteps. A popular isekai light novel and anime series in the 2000s was Zero no Tsukaima, where the male lead Saito is from modern Japan and is summoned to a fantasy world by the female lead Louise.
The genre eventually became so popular that in 2016, a Japanese short story contest organized by Bungaku Free Market and Shōsetsuka ni Narō banned any isekai entries. The publisher Kadokawa banned isekai stories as well in their own anime/manga-style novel contest in 2017.
In 2017, Geode conducted a poll in Japan asking people to name their favourite isekai anime. The top ten were Spirited Away, Pop in Q, Sword Art Online, Magic Knight Rayearth, Re:Zero, The Twelve Kingdoms, KonoSuba, World Trigger, Kyo Kara Maoh! and Gate.