Contrefort (buttress)

A buttress is a relatively short piece of wall, perpendicular to and built against a church wall.
The purpose of a buttress is to distribute the horizontal forces from the roof to the foundation.
In the picture the contreforts built against the walls of the Église de-la-Sainte-Trinité, Sainte-Croix et Notre-Dame in Anzy-le-Duc.

Église de la Sainte-Trinité, Sainte-Croix et Notre-Dame - Anzy-le-Duc