world line
Physics term: the path of an object in spacetime. Used in relativity and theoretical physics.
In relativity theory, the motion of an object is represented as a world line.
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.
In this game, the story branches into different parallel worlds depending on your choices.
He claims he came from an alternative reality.
Refers to a different, often fantasy world, not necessarily a parallel timeline. 世界線 implies a branching or alternative version of the same world.
Compound of 世界 (world) and 線 (line). The physics sense is a direct translation of the English term 'world line'. The colloquial fiction sense likely arose from its use in science fiction and visual novels.