a. Anatomy Any of various distensible membranous sacs, such as the urinary bladder or the swim bladder, that serve as receptacles for fluid or gas.
b. Medicine A blister, pustule, or cyst filled with fluid or air; a vesicle.
c. An item resembling one of the membranous sacs in animals: the bladder of a buoyancy compensator.
2. Botany Any of various hollow or inflated saclike organs or structures, such as the floats of certain seaweeds or the specialized traps of bladderworts.
[Middle English bladdre, from Old English blǣdre; see bhlē- in the Appendix of Indo-European roots.]
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