An elongated tethered balloon or blimp with cables or net suspended from it to deter enemy planes that are flying low WordNet 3.0 & Open English WordNet

A barrage balloon is a type of airborne barrage, a large uncrewed tethered balloon used to defend ground targets against aircraft attack, by raising aloft steel cables which pose to hostile aircraft a severe risk of collision, making the attacker's approach difficult and hazardous. Wikipedia

A tethered balloon used as an obstacle to attacking aircraft Wikipedia Disambiguation

Large balloon tethered with metal cables Wikidata

A tethered balloon, attached to the ground with a metal cable, intended to deter low-flying enemy aircraft; used especially in World War II. Wiktionary

Tethered balloon for deterring enemy aircraft. Wiktionary (translation)