The Collaborative International Dictionary of English v.0.48:

Gill \Gill\ (g[i^]l), n. [Dan. gi[ae]lle, gelle; akin to Sw.
   g[aum]l, Icel. gj["o]lnar gills; cf. AS. geagl, geahl, jaw.]
   1. (Anat.) An organ for aquatic respiration; a branchia.
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            Fishes perform respiration under water by the gills.
                                                  --Ray.
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   Note: Gills are usually lamellar or filamentous appendages,
         through which the blood circulates, and in which it is
         exposed to the action of the air contained in the
         water. In vertebrates they are appendages of the
         visceral arches on either side of the neck. In
         invertebrates they occupy various situations.
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   2. pl. (Bot.) The radiating, gill-shaped plates forming the
      under surface of a mushroom.
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   3. (Zool.) The fleshy flap that hangs below the beak of a
      fowl; a wattle.
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   4. The flesh under or about the chin. --Swift.
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   5. (Spinning) One of the combs of closely ranged steel pins
      which divide the ribbons of flax fiber or wool into fewer
      parallel filaments. [Prob. so called from F. aiguilles,
      needles. --Ure.]
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   Gill arches, Gill bars. (Anat.) Same as Branchial
      arches.

   Gill clefts. (Anat.) Same as Branchial clefts. See under
      Branchial.

   Gill cover, Gill lid. See Operculum.

   Gill frame, or Gill head (Flax Manuf.), a spreader; a
      machine for subjecting flax to the action of gills.
      --Knight.

   Gill net, a flat net so suspended in the water that its
      meshes allow the heads of fish to pass, but catch in the
      gills when they seek to extricate themselves.

   Gill opening, or Gill slit (Anat.), an opening behind and
      below the head of most fishes, and some amphibians, by
      which the water from the gills is discharged. In most
      fishes there is a single opening on each side, but in the
      sharks and rays there are five, or more, on each side.

   Gill rakes, or Gill rakers (Anat.), horny filaments, or
      progresses, on the inside of the branchial arches of
      fishes, which help to prevent solid substances from being
      carried into gill cavities.
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WordNet (r) 3.0 (2006):

gill net
    n 1: a flat fishnet suspended vertically in the water to
         entangle fish by their gills