(n.) A small hole, as the stop in a flute; a vent.

CAT aerospace aerosphere air duct air passage air pocket air shaft air tube airspace airway armhole blowhole breathing hole bullet-hole bump bunghole ceiling cringle crosswind deadeye empty space eye eyelet favorable wind fog front gasket grommet guide head wind high-pressure area hole ionosphere jetstream keyhole knothole loop loophole louver louverwork low-pressure area manhole mousehole naris nostril overcast peephole pigeonhole pinhole placket placket hole pocket porthole punch-hole roughness shaft soup space spilehole spiracle stratosphere substratosphere tail wind tap touchhole transom tropopause troposphere trough turbulence vent ventage venthole ventiduct ventilating shaft ventilator visibility visibility zero wind tunnel