dressing room; powder room
A room for applying makeup and dressing, often in a theater or traditional setting. The gloss 'lavatory' is a dated euphemism; the primary meaning is a room for grooming.
There is a dressing room next to the backstage room.
She got ready in the dressing room.
楽屋 is a backstage room for performers to relax, while 化粧部屋 specifically refers to a room for applying makeup and dressing.
化粧室 is a common euphemism for a restroom, whereas 化粧部屋 is a literal dressing room, not a toilet.
Compound of 化粧 (makeup) and 部屋 (room).