noun

1. (usually with article) A bizarre world, where everyday rules do not apply, or a way into such a world.

   “These mushrooms will take you down the rabbit hole, man.”

2. (by extension) A time-consuming tangent or detour, often from which it is difficult to extricate oneself.

3. The initial clue that leads to an alternate reality game.

Synonyms: looking glass, trailhead