From Planet X
1. Any of various distensible membranous sacs, such as the urinary bladder or the swim bladder, that serve as receptacles for fluid or gas. 2. the bladder of a football. 3. Any of various hollow or inflated saclike organs or structures, such as the floats of certain seaweeds or the specialized traps of bladderworts. 4. A blister, pustule, or cyst filled with fluid or air; a vesicle.