A buttress is a support--usually made of brick or stone--built against a wall to support or reinforce it. A flying buttress is a free-standing buttress attached to the main structure by an arch or a half-arch.
butress
"At the moment when his thought was thus fixed upon the priest, while the daybreak was whitening the flying buttresses, he perceived on the highest story of Notre-Dame, at the angle formed by the external balustrade as it makes the turn of the chancel, a figure walking." --Victor Hugo, The Hunchback of Notre-Dame


