a distensible membranous sac (usually containing liquid or gas)
source: WordNet 3.0
a bag that fills with air
source: WordNet 3.0
A bag or sac in animals, which serves as the receptacle of some fluid; as, the urinary bladder; the gall bladder; -- applied especially to the urinary bladder, either within the animal, or when taken out and inflated with air.