Swim bladder

from WordNet (r) 3.0 (2006)

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

from The Collaborative International Dictionary of English v.0.48

Swim \Swim\, n.
   1. The act of swimming; a gliding motion, like that of one
      swimming. --B. Jonson.
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   2. The sound, or air bladder, of a fish.
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   3. A part of a stream much frequented by fish. [Eng.]
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   {Swim bladder}, an air bladder of a fish.

   {To be in the swim}, to be in a favored position; to be
      associated with others in active affairs. [Colloq.]
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