noun
fitted sheet; box sheet
A sheet with elasticated corners designed to fit snugly over a mattress. The term 'box sheet' is a direct loan from English but less common than 'fitted sheet' in English-speaking countries.
新しいボックスシーツを買ったので、ベッドにぴったりフィットした。
I bought a new fitted sheet, and it fits the bed perfectly.
このボックスシーツは洗濯しても縮ま
This fitted sheet doesn't shrink even after washing.