Buttress. A mass of masonry or brickwork projecting from or built against a wall to give additional strength, usually to counteract the lateral thrust of an arch, roof or vault.
Flying Buttress. An arch or half-arch transmitting the thrust of a vault or roof from the upper part of a wall to an outer support or buttress.
-- John Fleming, Hugh Honour & Nikolaus Pevsner, 1991
The Penguin Dictionary of Architecture
Penguin Books, London, England