What is the architectural support added to the exterior of a gothic building that forms an arch?

The architectural support added to the exterior of a Gothic building that forms an arch is known as a "flying buttress." This structure transfers the weight of the roof and walls away from the building, allowing for higher ceilings and larger Windows, often filled with stained glass. The flying buttress is both functional and decorative, contributing to the characteristic verticality and lightness of Gothic architecture.