The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. A sac or bladder full of air in an animal or plant; also
      an air hole in a casting.
      [1913 Webster]

WordNet (r) 3.0 (2006):

air bladder
    n 1: an air-filled sac near the spinal column in many fishes
         that helps maintain buoyancy [syn: air bladder, swim
         bladder, float]