The flying buttress (arc-boutant, arch buttress) is a specific form of buttress composed of an arch that extends from the upper portion of a wall to a pier of great mass, in order to convey to the ground the lateral forces that push a wall outwards, which are forces that arise from vaulted ceilings of stone and from wind-loading on roofs.

The namesake and defining feature of a flying buttress is that it is not in contact with the wall at ground level, unlike a traditional buttress, and transmits the lateral forces across the span of intervening space between the wall and the pier. To provide lateral support, flying-buttress systems are composed of two parts: (i) a massive pier, a vertical block of masonry situated away from the building wall, and (ii) an arch that bridges the span between the pier and the wall — either a segmental arch or a quadrant arch — the flyer of the flying buttress.