Wiktionary
balloon sail
n. (context nautical English) A large, light foresail sometimes used instead of a jib.
WordNet
balloon sail
n. any light loose sail
Usage examples of "balloon sail".
The Hadji was about-forty feet long and essentially consisted of two sharp-prowed bamboo hulls fastened together with a platform, a bowsprit with a balloon sail and a single mast, fore-and-aft rigged, with sails of woven bamboo fibers.