noun
vent; ventilation opening
A physical opening or duct for ventilation, such as in a wall, ceiling, or appliance.
台所の換気口から冷たい風が入ってくる。
Cold air comes in through the kitchen vent.
浴室の換気口を掃除してください。
Please clean the bathroom ventilation opening.
換気扇 is a ventilation fan (active device), while 換気口 is a passive opening or vent.
Compound of 換気 (ventilation) + 口 (opening/mouth).