In Philip Pullman’s fantasy novel The Subtle Knife, the knife that cuts windows between worlds is called the subtle knife.
The knife is an ancient, finely crafted weapon that can make openings between parallel worlds. Its edge is so precise that it can cut through the fabric separating realities, while another edge can seal those openings. This ability makes the object far more important than an ordinary blade.
The knife is discovered in the world of Cittàgazze and becomes closely associated with Will Parry, who carries it during much of the story. Its use also creates serious consequences, because uncontrolled windows allow dangerous beings called Specters to move between worlds.
The Subtle Knife is the second novel in Pullman’s His Dark Materials trilogy, following Northern Lights and preceding The Amber Spyglass. The title refers to the artifact rather than to a character or location.