Source : Webster's Revised Unabridged Dictionary (1913)

Air bladder \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.

Source : WordNet®

air bladder
     n : an air-filled sac near the spinal column in many fishes that
         helps maintain buoyancy [syn: {swim bladder}]