a membranous sac in animals that serves as the receptacle of a liquid or contains gas, esp urine the urinary bladder
a hollow balloon-like part in e.g. a frond of seaweed; a vesicle (1a)
a bag filled with a liquid or gas (e.g. the air-filled rubber bag inside a football)
[Old English blǣdre; related to Old English blāwan a blow1]