Threshold
The boundary between ordinary and sacred worlds that must be crossed for transformation to occur.
The threshold is the liminal space between worlds — the point of no return in the hero's journey. Crossing the threshold means leaving behind the familiar and entering the unknown. Threshold guardians test whether the initiate is ready to proceed. Once crossed, the threshold cannot be uncrossed; the initiate is changed even if they later return. In film, thresholds are often literalized: the rabbit hole, the red pill, the wardrobe into Narnia, the portal into another world. The threshold crossing commits the protagonist to transformation.