(Ital., from Lat. copula, small cask or vault, cupa, tub), a term, in architecture, for a spherical or spheroidal covering to a building, or to any part of it. In fortification the word is used of a form of armoured structure, in which guns or howitzers are mounted. It is a low flat turret resembling an overturned saucer and showing little above the ground except the muzzles of the guns. See for details and illustrations Fortification And Siegecraft; also Ordnance.