loungewear; indoor clothes; comfy home clothes
Casual, comfortable clothing worn at home; distinct from pajamas or going-out clothes.
帰宅したらすぐに部屋着に着替える。
I change into loungewear as soon as I get home.
休日は一日
On my days off, I spend the whole day in my comfy home clothes.
部屋着 refers narrowly to clothes worn indoors at home, while 普段着 covers everyday casual clothes worn both inside and outside.
部屋着 means clothes worn for relaxing at home and isn't necessarily for sleeping, whereas 寝間着 is specifically sleepwear.
Compound of 部屋 (room) and 着 (clothing). A straightforward modern coinage for indoor loungewear.