P-Balloons are power-ups in Super Mario World and its remake. If Mario or Luigi touch it, they will inflate like a balloon, enabling them to float about the screen and reach otherwise inaccessible areas. They have their own level in the Special World called Tubular. Taking damage reverts them to their normal form. It is also Mario's power flower ability in Super Mario 64 DS. If you Ground Pound you turn to normal.
In the Super Mario World cartoon the P-Balloon stays the same.