A room for washing one's face and brushing teeth. 'Senmenshitsu' is commonly used in public facilities or hospitals; at home people often say 'senmenjo'.
待合室の隣に洗面室があるので、手を洗ってください。
There is a washroom next to the waiting room, so please wash your hands.
ホテルの洗面室は広く、アメニティも充実していた。
The hotel's washroom was spacious and well equipped with amenities.
病院の洗面室は消毒用の設備が整っていることが多い。
Hospital washrooms often have facilities for disinfection.