Source : Webster's Revised Unabridged Dictionary (1913)

Swim \Swim\, n.
   1. The act of swimming; a gliding motion, like that of one
      swimming. --B. Jonson.

   2. The sound, or air bladder, of a fish.

   3. A part of a stream much frequented by fish. [Eng.]

   {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.]

Source : WordNet®

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