- a vesicle or sac containing air.
- Also called gas bladder, swim bladder. Ichthyology. 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.
- Also called: swim bladder ichthyol an air-filled sac, lying above the alimentary canal in bony fishes, that regulates buoyancy at different depths by a variation in the pressure of the air
- any air-filled sac, such as one of the bladders of seaweeds
- An air-filled sac in many fish that helps maintain buoyancy or, in some species, helps in respiration, sound production, or hearing. Also called swim bladder
- See float.