1. An extremely large bag filled with a lighter-than-air gas (such as helium) and used as a form of air transport, carrying passengers or equipment in a suspended basket or gondola: We decided to take a ride in a balloon to see the countryside from a different perspective.
2. A small sac, typically of rubber or Mylar, inflated with air or a lighter-than-air gas often used in celebrations: The teacher explained that a rubber balloon may cause harm to birds and encouraged the children not to use them for their parties.
3. A rounded outline with a point directed toward a character in a cartoon that encloses the text of the character's speech or thought: Cartoonists often use one balloon after the other to express each character's spoken words or thoughts.

A sac that is inserted into a body cavity or tube and distended with air or gas for therapeutic purposes; such as, with angioplasty: The doctor explained that he would inflate a balloon within the blood vessel so it would expand and permit the normal flow of blood again.

To become bigger quickly: The monthly report indicated that Dick's credit card debt would balloon at least three times more than normal and, in addition, there would be the ballooning costs of a college education to consider.

A final payment that is much larger than any earlier payment made on a debt: Trisha agreed to pay $1,000 a year for ten years and then make a balloon payment of $60,000 at the end of the period.