Arch. The spanning of an opening by means other than that of a lintel. The most primitive form was made by corbelling. True arches are curved and so constructed with wedge-shaped blocks over the opening that the downward thrust of the weight of their own material and of that above is converted into outward thrusts resisted by the flanking material. There are many different types.
-- John Fleming, Hugh Honour & Nikolaus Pevsner, 1991
The Penguin Dictionary of Architecture
Penguin Books, London, England