The Oxford dictionary gives only two meanings for the word "bikini": the atoll and the 2-piece swimsuit. But I think that sometimes this word refers to the underpants only and not the entire suit. Is it a correct use or am I missing something?
Learn English – Can the word “bikini” refer to the bottom part of the bikini swimsuit
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If you are only talking about swimsuits, the most widespread term (at least in the US) is bikini bottom. Bikini tops and bottoms can be sold separately, but they are considered pieces of one outfit, similar to shirts and pants. If you use the plural "bikinis" with a singular subject, it does refer to just the bottoms. For a feminine subject, it almost always refers to bikini panties, but for a man it might be unclear whether it was a swimsuit or underwear depending on the dialect.
Some folks refer to male bikini style swimsuits as "speedos", the most popular brand of that style in the past. The Speedos USA site refers to that style as "briefs", not bikinis. Thinking about it, I hang out with a group of folks that's pretty diverse when it comes to gender identity and sexual orientation, so "briefs" might be much more mainstream than "bikinis".
Depending on the context, it can be ambiguous for women as to whether you're referring to the panties or to both pieces of the swimsuit. For example, "She wore bikinis every day." could be interpreted either way. Most online stores have a "bikinis" category of women's underwear, while the male category is usually "bikini briefs".
Bikini bottoms on swimsuits have a certain shape of leg hole. There are other styles swimsuit of bottoms.
If you say "The girl was wearing a bikini." it will be assumed that she was wearing both pieces of a bikini swimsuit. If you say "The woman was wearing a bikini bottom.", it's unclear what type of top she might or might not have been wearing, but it will be clear what sort of pants she had on. If you are talking about a man wearing bikini bottoms, you can just say "bikinis". For example, "Most of the men were wearing swim trunks, but Joe was wearing bikinis." If you say "Joe was wearing a bikini", some folks might get a picture of a man wearing both pieces of a women's bikini.
Monokinis these days (in the US at least) are one piece swimsuits that cover both top and bottom, but are more revealing than a typical one piece swimsuit and intended to be very sexy. A regular one-piece swimsuit that is styled to be useful for swimming instead of laying on the beach looking sexy is usually called a "tank" swimsuit.