1. a vesicle or sac containing air.
2. Also called gas bladder, swim bladder. Ichthyol.a gas-filled sac located against the roof of the body cavity of most bony fishes, originally functioning only as a lung, now serving in many higher fishes to regulate hydrostatic pressure.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.