also: ベッドパット · ベットパット · ベットパッド
mattress pad; mattress topper; underpad
A padded layer placed on top of a mattress for comfort, protection, or warmth. Wasei-eigo from 'bed pad'.
wasei-eigo (Japanese-coined English), from bed pad
After I bought a new mattress pad, my sleep quality improved.
This mattress pad is machine-washable.