What is "vent feather"

Vent \Vent\, n. [OE. fent, fente, a slit, F. fente a slit, cleft, fissure, from fendre to split, L. findere; but probably confused with F. vent wind, L. ventus. See Fissure, and cf. Vent to snuff.]

1. A small aperture; a hole or passage for air or any fluid to escape; as, the vent of a cask; the vent of a mold; a volcanic vent.

   Look, how thy wounds do bleed at many vents.
   --Shak.

   Long't was doubtful, both so closely pent, Which first should issue from the narrow vent.
   --Pope.

2. Specifically:

   1. (Zo["o]l.) The anal opening of certain invertebrates and fishes; also, the external cloacal opening of reptiles, birds, amphibians, and many fishes.

   2. (Gun.) The opening at the breech of a firearm, through which fire is communicated to the powder of the charge; touchhole.

   3. (Steam Boilers) Sectional area of the passage for gases divided by the length of the same passage in feet.

3. Fig.: Opportunity of escape or passage from confinement or privacy; outlet.

4. Emission; escape; passage to notice or expression; publication; utterance. Without the vent of words. --Milton. Thou didst make tolerable vent of thy travel. --Shak. To give vent to, to suffer to escape; to let out; to pour forth; as, to give vent to anger. To take vent, to escape; to be made public. [R.] Vent feather (Zo["o]l.), one of the anal, or crissal, feathers of a bird. Vent field (Gun.), a flat raised surface around a vent. Vent piece. (Gun.)

   1. A bush. See 4th Bush, n., 2.

   2. A breech block.