Pronunciation: (bu-lOOn'),[key] —n. 1. a bag made of thin rubber or other light material, usually brightly colored, inflated with air or with some lighter-than-air gas and used as a children's plaything or as a decoration.
2. a bag made of a light material, as silk or plastic, filled with heated air or a gas lighter than air, designed to rise and float in the atmosphere and often having a car or gondola attached below for carrying passengers or scientific instruments.
3. (in drawings, cartoons, etc.) a balloon-shaped outline enclosing words represented as issuing from the mouth of the speaker.
4. an ornamental ball at the top of a pillar, pier, or the like.
5. a large, globular wineglass.
6. Chem. Now Rare.a round-bottomed flask.
—v.i. 1. to go up or ride in a balloon.
2. to swell or puff out like a balloon.
3. to multiply or increase at a rapid rate: Membership has ballooned beyond all expectations.
—v.t. to fill with air; inflate or distend (something) like a balloon.
—adj. 1. puffed out like a balloon: balloon sleeves. 2. Finance.(of a loan, mortgage, or the like) having a payment at the end of the term that is much bigger than previous ones.