A buttress is a support built against a wall to support or reinforce it.
One of the greatest innovations of the Gothic era was the "flying buttress" system of structural support. Attaching to the external walls, these free-standing, arched buttresses allowed builders to construct high cathedrals with soaring interior spaces.
Buttresses remain an important structural element in modern buildings. An innovative system of Y-shaped buttresses allowed the Burj Khalifa in Dubai to reach record-breaking height.
"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