loungewear; indoor clothes; comfy home clothes
Casual, comfortable clothing worn at home; distinct from pajamas or going-out clothes.
I change into loungewear as soon as I get home.
On my days off, I spend the whole day in my comfy home clothes.
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.