noun
bathtub cover; bathtub lid
A flat or folding cover placed over a bathtub to keep the water warm when not in use.
お風呂に入った後は、風呂ふたを閉めてください。
Please put the bathtub cover on after taking a bath.
風呂ふたを開けると、まだ湯気が立っていた。
When I opened the bathtub lid, steam was still rising.
Compound of 風呂 (furo, 'bath') and ふた (futa, 'lid'). The kana spelling ふた is common for the lid component.