1. (n.) The nostril, or one of the nostrils, of whales, porpoises, and allied animals.

2. (n.) One of the external openings communicating with the air tubes or tracheae of insects, myriapods, and arachnids. They are variable in number, and are usually situated on the sides of the thorax and abdomen, a pair to a segment. These openings are usually elliptical, and capable of being closed.

3. (n.) A tubular orifice communicating with the gill cavity of certain ganoid and all elasmobranch fishes. It is the modified first gill cleft.

4. (n.) Any small aperture or vent for air or other fluid.

air duct air hole air passage air shaft air tube airway armhole avenue blowhole breathing hole bullet-hole bunghole channel chute cringle deadeye debouch door egress emunctory escape estuary exhaust exit eye eyelet floodgate flume gasket grommet guide keyhole knothole loop loophole louver louverwork manhole mousehole naris nostril opening out outcome outfall outgate outgo outlet peephole pigeonhole pinhole placket placket hole pore port porthole punch-hole sally port shaft sluice spilehole spout tap touchhole transom vent ventage venthole ventiduct ventilating shaft ventilator vomitory way out weir wind tunnel