世せ界かい線せん

noun

world line

Physics term: the path of an object in spacetime. Used in relativity and theoretical physics.

• 相そう対たい性せい理り論ろんでは、物ぶっ体たいの運うん動どうを世せ界かい線せんとして表あらわす。

  In relativity theory, the motion of an object is represented as a world line.

noun

parallel universe; alternative reality

Colloquial use in fiction (especially sci-fi and visual novels) to refer to a parallel world or timeline. Often used in contexts involving time travel or branching realities.

• このゲームでは、選せん択たくによって異ことなる世せ界かい線せんに分ぶん岐きする。

  In this game, the story branches into different parallel worlds depending on your choices.

• 彼かれは別べつの世せ界かい線せんから来きたと言いっている。

  He claims he came from an alternative reality.