vent; ventilation opening
A physical opening or duct for ventilation, such as in a wall, ceiling, or appliance.
Cold air comes in through the kitchen vent.
Please clean the bathroom ventilation opening.
換気口 specifically denotes a mechanical ventilation grille in a building, whereas 風穴 is broader and can describe natural or informal air holes too.
Standard kanji spelling; 口 is the usual character for an opening or vent.
Alternate spelling using 孔 (hole); less common but still seen in technical contexts.
Compound of 換気 (ventilation) + 口 (opening/mouth).