Isekai, literally "different world," is the genre in which a protagonist from ordinary life is transported to, reborn into, or otherwise stranded in another world, usually a fantasy one. The journey between worlds is the premise, not a plot twist.
The structure is older than the label. Stories of ordinary people carried into fairy realms exist in folklore worldwide, and modern Japanese fiction ran the play for decades before the term settled. The contemporary boom arrived through web novel sites in the 2000s and 2010s, where amateur authors published serialized other-world stories in enormous numbers and the most popular graduated to print and anime.
The genre has recognizable machinery: a protagonist who arrives with modern knowledge or an unusual advantage, a world that often runs on game-like rules, and a reset of social position. Reincarnation into a new life in the other world is the most common modern variant, and the long, descriptive titles of web novels often summarize the entire premise.
Fans argue the borders. A character trapped inside a game world is adjacent rather than strictly isekai, since no new world is involved, and a story that begins in the fantasy world with no transfer at all is not isekai however similar it feels. The transfer is the definition.
In use"A office worker who wakes up as a child in a village with magic has crossed into an isekai premise."