noun
another world; parallel universe; isekai
Primarily used in fantasy, light novels, anime, and manga to refer to a world different from our own, often one the protagonist is transported to. The term 'isekai' is also used as a genre label in English.
彼はトラックに轢かれて異世界に転生した。
He was hit by a truck and reincarnated in another world.
最近のアニメは異世界ものが多い。
Many recent anime are isekai stories.
パラレルワールド (parallel world) is a direct loanword and can be used in both science fiction and fantasy, but 異世界 is more strongly associated with Japanese fantasy fiction.
Compound of 異 (i, 'different') + 世界 (sekai, 'world'). The term became widely popular through Japanese fantasy fiction and is now a recognized genre label.