What architectural feature supports the walls of Gothic cathedrals from outside?

The architectural feature that supports Gothic cathedral walls from outside is the flying buttress.

A flying buttress is an exterior support system made of a masonry pier connected to the upper wall by an arched “flyer.” The arch carries sideways forces from the stone vaults and roof outward to the pier, which transfers them safely into the ground. Unlike a conventional buttress, it does not run continuously from the wall to the ground.

This innovation helped Gothic architects build taller, lighter churches. Because the walls no longer needed to be massively thick, builders could open larger areas for stained-glass windows and clerestories. The result was the height, light, and vertical emphasis associated with Gothic interiors.

Flying buttresses are often confused with spires, rose windows, or gargoyles. Spires crown towers, rose windows admit colored light, and gargoyles commonly function as waterspouts or decoration. The flying buttress is primarily structural, although medieval builders often embellished its piers with pinnacles and carvings. Notre-Dame de Paris had some of the earliest prominent examples in a Gothic cathedral, dating to about 1180.