Is there a noun that means the piece of cloth that some women wear around their waist when they are in bikini?
In Britain they are known variously as a sarong (from Malay national garment), pareo (from pareu - Tahitian national language, first used in English in 1769), kimono (from Japanese), or simply as wrap, or cover-up. All those terms appear on Amazon
The Marks & Spencers word seems to be sarong, which suggests it is the more usual term in Britain. They don't seem to have heard of a pareo.
These are pictures of the real Sarong Kebaya which is a Malay national dress. It is the uniform of the female flight attendants on both MAS (Malaysian Airlines) and SIA (Singapore Airlines). They are as different from beach wraps as are Japanese kimonos.
I would call it a lavalava, particularly if worn by a Polynesian and definitely if it were a man wearing it.
You probably mean a pareo?
Here's an example of one: