I believe that element is called flying buttresses.
Flying buttresses support the walls of a building and given that they are quite slanted, they seem as if they are forming an arch with that particular wall they are leaned against. I've attached an image of them for you to see what I mean.
Flying buttresses support the walls of a building and given that they are quite slanted, they seem as if they are forming an arch with that particular wall they are leaned against. I've attached an image of them for you to see what I mean.