The item whose presence always pushes the story forward toward the climax

The Catalyst creates breakthroughs and accelerates the conflict, or conflict resolution, of the throughline it is in. In both the Overall and Main vs. Impact Stories there occur dramatic "log-jams" when things seem to be approaching a halt. This is when the Catalyst is necessary, for its introduction will either solve the puzzle that's holding things up or else make the puzzle seem suddenly unimportant so the story can continue.