A room with washing facilities, such as a bathroom or powder room. Often synonymous with 洗面所, though 洗面室 may sound slightly more formal or architectural.
See also: 洗面所 (せんめんじょ)
The washroom in this hotel is very clean.
Compound of 洗面 (washing one's face) and 室 (room). The exact historical derivation is uncertain, but the spelling is conventionally associated with a room for washing.