bal·loon 
(bə-ln)
(bə-ln)n.
1.
a. A flexible bag designed to be inflated with hot air or with a gas, such as helium, that is lighter than the surrounding air, causing it to rise and float in the atmosphere.
b. Such a bag with sufficient capacity to lift and transport a suspended gondola or other load.
c. Such a bag shaped like a figure or object when inflated; an inflatable.
2. A usually round or oblong inflatable rubber bag used as a toy or decoration.
3. Medicine An inflatable device that is inserted into a body cavity or structure and distended with air or gas for therapeutic purposes, such as angioplasty.
4.
a. See speech bubble.
b. See thought bubble.
5. A balloon payment.
v. bal·looned, bal·loon·ing, bal·loons
v.intr.
1. To ascend or ride in a balloon.
2. To expand or swell out like a balloon. See Synonyms at bulge.
3. To increase or rise quickly: expenses ballooning out of control.
v.tr.
To cause to expand by or as if by inflating: unforeseen expenditures that ballooned the deficit.
adj.
Suggestive of a balloon, as in shape: balloon curtains.
[French ballon, from Italian dialectal ballone, augmentative of balla, ball, of Germanic origin; see bhel-2 in the Appendix of Indo-European roots.]
bal·loonist n.