What is the heavy arched stone support built on the outside of walls?

The heavy arched stone support built on the outside of walls is called a "buttress." Buttresses are used to provide additional stability and support to structures, particularly in Gothic architecture, where they help counteract the lateral forces acting on the walls. They can be both functional and decorative, often featuring intricate designs. Flying buttresses, a specific type, extend away from the wall and are commonly seen in cathedrals.