noun
drain outlet; drain opening; overflow drain
Refers to the opening or outlet where water drains away, such as in a sink, bathtub, or floor. 'Overflow' refers to the secondary drain hole near the top of a bathtub or sink to prevent overflowing.
台所の排水口にゴミが詰まっている。
The kitchen drain is clogged with debris.
お風呂の排水口の掃除を忘
Don't forget to clean the bathtub drain.
排水管 is the drain pipe, whereas 排水口 is the opening where water enters the drainage system.
Compound of 排水 (drainage) and 口 (opening, mouth).