Barrage balloons are large balloons which are fixed to the ground by strong steel cables. They are used in wartime, when the cables are intended to destroy low-flying enemy aircraft.
barrage balloon in British English
noun
one of a number of tethered balloons with cables or net suspended from them, used to deter low-flying air attack
barrage balloon in American English
(bəˈrɑʒ; bəˈrɑdʒ)
any of a number of anchored balloons with cables or nets attached for entangling low-flying attacking airplanes