air blad·der
 1. Anat. An air sac, sometimes double or variously lobed, in the visceral cavity of many fishes. It originates in the same way as the lungs of air-breathing vertebrates, and in the adult may retain a tubular connection with the pharynx or esophagus.
 2. A sac or bladder full of air in an animal or plant; also an air hole in a casting.
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 air bladder
      n : an air-filled sac near the spinal column in many fishes that
          helps maintain buoyancy [syn: swim bladder]