A bikini or two piece is a women's

A bikini or two piece is a women's swimsuit with two parts, one covering the breasts, the other the groin (and optionally the buttocks), leaving an uncovered area between the two (optionally in the case of the Tankini). It is often worn in hot weather or while swimming. The shapes of both parts of a bikini resemble women's underwear, and the lower part can range from revealing thong or g-string to briefs and modest square-cut shorts. Merriam–Webster's Collegiate Dictionary (11th edition) describes the bikini as "a woman's scanty two-piece bathing suit", "a man's brief swimsuit" and "a man's or woman's low-cut briefs".

The bikini, which shocked when it appeared on French beaches in 1947, dates back to antiquity, in Çatalhöyük and the Greco-Roman world.The modern bikini was invented by French engineer Louis Réard in 1946. He named it after Bikini Atoll in the Pacific, the site of the Operation Crossroads nuclear weapon tests in July that year. The reasoning was that the burst of excitement created by it would be like a nuclear device. The monokini, a bikini variant, is a back formation from bikini, interpreting the first syllable as the Latin prefix bi- meaning "two" or "doubled", and substituting for it mono- meaning "one".Jacques Heim called his bikini precursor the Atome, named for its size, and Louis Réard claimed to have "split the Atome" to make it smaller.

From a 1949 Los Angeles Times report: "The bathing beauty queen—blond Bebe Shopp, 18, of Hopkins, Minn.—got an enthusiastic welcome in Paris, but she said she hasn't changed her mind about French swim suits. ... 'I don't approve of Bikini suits for American girls,' Bebe told her French interviewers. 'The French girls can wear them if they want to, but I still don't approve of them on American girls."

The bikini is perhaps the most popular female beachwear around the globe, according to French fashion historian Olivier Saillard due to "the power of women, and not the power of fashion". As he explains, "The emancipation of swimwear has always been linked to the emancipation of women."By the mid 2000s bikinis had become a US$811 million business annually, according to the NPD Group, a consumer and retail information company.The bikini has boosted spin-off services like bikini waxing and the sun tanning industries

The history of the bikini is a checkered one. The earliest evidence of a bikini-like costume dates back to the Chalcolithic era, as the mother-goddess of Çatalhöyük, a large ancient settlement in southern Anatolia, is depicted astride two leopards wearing garb akin to a modern bikini.[1] Two-piece garments worn by women for athletic purposes are on Greek urns and paintings dating back to 1400 BC.Active women of ancient Greece wore a breastband called a mastodeton or an apodesmos, which continued to be used as an undergarment in the Middle Ages.[9] While men in ancient Greece abandoned the perizoma, partly high-cut briefs and partly loincloth, women performers and acrobats continued to wear it. Artwork dating back to the Diocletian period (286-305 AD) in Villa Romana del Casale, Sicily depicts women in garments resembling bikinis in mosaics on the floor.The images of ten women, dubbed the "Bikini Girls", exercising in clothing that would pass as bikinis today, are the most replicated mosaic among the 37 million colored tiles at the site. Archeological finds, especially in Pompeii, show the Roman goddess Venus wearing a bikini. A statue of Venus in a bikini was found in cupboard in the southwest corner in Casa della Venere, others were found in the front hall.[14] A statue of Venus was recovered from the tablinum of the house of Julia Felix,[15] and another from an atrium at the garden at Via Dell'Abbondanza.

The modern bikini started to emerge again in 1907, when Australian swimmer and performer Annette Kellerman was arrested on a Boston beach for wearing a form-fitting one-piece which became an accepted swimsuit for women by 1910. Pictures of her were produced as evidence in the Esquire magazine versus United States Postmaster General legal battle over indecency in 1943. In 1913, inspired by the introduction of females into Olympic swimming, the designer Carl Jantzen made the first functional two-piece swimwear, a close-fitting one-piece with shorts on the bottom and short sleeves on top. By the 1930s, necklines plunged at the back, sleeves disappeared and sides were cut away. Hollywood endorsed the new glamour with films such as Neptune's Daughter in which Esther Williams wore provocatively named costumes such as "Double Entendre" and "Honey Child".With new materials like lastex and nylon, by 1934 the swimsuit started hugging the body and had shoulder straps to lower for tanning.Burlesque and vaudeville performers wore two-piece outfits in the 1920s, and in 1932 French designer Madeleine Vionnet offered an exposed midriff in an evening gown.

By the early 1940s two-piece swimsuits were frequent on American beaches. Hollywood stars like Ava Gardner, Rita Hayworth and Lana Turner tried similar swimwear or beachwear.[21] Pin ups of Hayworth and Esther Williams in the costume were widely distributed. Finally, the modern bikini was introduced by French engineer Louis Réard and fashion designer Jacques Heim in Paris in 1946. Réard was a car engineer but by 1946 he was running his mother's lingerie boutique near Les Folies Bergères in Paris.Heim was working on a new kind of beach costume. It comprised two pieces, the bottom large enough to cover its wearer's navel. In May 1946, he advertised it as the world's "smallest bathing suit". Réard named his swimsuit the “bikini,” taking the name from the Bikini Atoll, one of a series of islands in the South Pacific where testing on the new atomic bomb was occurring that summer. Historians assume Reard termed his swimsuit the “bikini” because he believed its revealing style would create reactions among people similar to those created by America’s atomic bomb in Japan just one summer earlier.[1] Réard sliced the top off the bottoms and advertised it as "smaller than the smallest swimsuit".Réard could not find a model to wear his design. He ended up hiring Micheline Bernardini, a nude dancer from the Casino de Paris.That bikini, a string bikini with a g-string back of 30 square inches (194 cm²) of clothes with newspaper type printed across, was introduced on July 5 at Piscine Molitor, a public pool in Paris.Heim's design was the first worn on the beach, but clothing was given its name by Réard.