loungewear; indoor clothes
Comfortable clothing worn inside the home. Often contrasted with 部屋着 (room wear) or パジャマ (pajamas), but 室内着 can imply slightly more presentable home wear.
See also: 部屋着
I change into loungewear as soon as I get home.
In winter, I want loungewear made of warm material.
Compound of 室内 (indoor) + 着 (clothing). A straightforward modern coinage.