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.
Fishes perform respiration under water by the gills.
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.
2. pl. (Bot.) The radiating, gill-shaped plates forming the
under surface of a mushroom.
3. (Zool.) The fleshy flap that hangs below the beak of a
fowl; a wattle.
4. The flesh under or about the chin. --Swift.
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,
Gill arches, Gill bars. (Anat.) Same as Branchial
Gill clefts. (Anat.) Same as Branchial clefts. See under
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.
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.