balloon sail

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A comparatively large foresail used when going before the wind to supplement or replace a jib.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun nautical A large, light foresail sometimes used instead of a jib.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any light loose sail