A bikini is a two-piece swimsuit that features one piece on top that covers the breasts, and a second piece on the bottom: the front covering the pelvis but usually exposing the navel, and the back generally covering the intergluteal cleft and some or all of the buttocks. The size of the top and bottom can vary, from bikinis that offer full coverage of the breasts, pelvis, and buttocks, to more revealing designs with a thong or G-string bottom that covers only the mons pubis, but exposes the buttocks, and a top that covers only the areolae. Bikini bottoms covering about half the buttocks may be described as "Brazilian-cut".

The modern bikini swimsuit was introduced by French clothing designer Louis Réard in July 1946, and was named after the Bikini Atoll, where the first public test of a nuclear bomb had taken place four days before.

Due to its revealing design, the bikini was once considered controversial, facing opposition from a number of groups and being accepted only very slowly by the general public. In many countries, the design was banned from beaches and other public places: in 1949, France banned the bikini from being worn on its coastlines; in the 1950s and 1960s, bikinis were banned in many pools and beaches in West Germany (based on a 1932 decree that only allowed women to swim in public if they wore a swimming costume that completely covered the front of their upper body, including their breasts), and some communist groups condemned the bikini as a "capitalist decadence". The bikini also faced criticism from some feminists, who reviled it as a garment designed to suit men's tastes, and not those of women. Despite this backlash, however, the bikini still sold well throughout the mid to late 20th century.^{[citation needed]}

The bikini gained increased exposure and acceptance as film stars like Brigitte Bardot, Raquel Welch, and Ursula Andress wore it and were photographed on public beaches and seen in film. The minimalist bikini design became common in most Western countries by the mid-1960s as both swimwear and underwear. By the late 20th century, it was widely used as sportswear in beach volleyball and bodybuilding. There are a number of modern stylistic variations of the design used for marketing purposes and as industry classifications, including monokini, microkini, tankini, trikini, pubikini, skirtini, thong, and g-string. A man's single-piece brief swimsuit may also be called a bikini or "bikini brief", particularly if it has slimmer sides. Similarly, a variety of men's and women's underwear types are described as bikini underwear. The bikini has gradually gained wide acceptance in Western society. By the early 2000s, bikinis had become a US$811 million business annually, and boosted spin off services such as bikini waxing and sun tanning.

While the two-piece swimsuit as a design existed in classical antiquity, the modern design first attracted public notice in Paris on July 5, 1946.

In May 1946, Parisian fashion designer Jacques Heim released a two-piece swimsuit design that he named the Atome ('Atom') and advertised as "the smallest swimsuit in the world". Like swimsuits of the era, it covered the wearer's belly button, and it failed to attract much attention. French automotive engineer Louis Réard introduced a design he named the "Bikini", adopting the name from the Bikini Atoll in the Pacific Ocean, which was the colonial name the Germans gave to the atoll, borrowed from the Marshallese name for the island, Pikinni. Four days earlier, on 1 July 1946, the United States had initiated its first peacetime nuclear weapons test at Bikini Atoll as part of Operation Crossroads. Unlike the prior Trinity test, or most subsequent nuclear test series, the United States allowed both international observers and the global press to observe Crossroads, creating an intense international interest in the new weapon and its testing. Réard never explained why he chose the name "Bikini" for the swimsuit. Various motivations have been attributed to his choosing of the name, including the idea that he hoped it would create "explosive commercial and cultural reaction" similar to the explosion at Bikini Atoll, that it was meant to be associated with the "exotic allure of the tropical Pacific", from the "comparison of the effects of a scantily clad woman to the atomic bomb," and the idea that Reard's design had out-done Heim's design and "split the atome". Réard's advertising slogan was that the Bikini was "smaller than the smallest bathing suit in the world." The swimsuit's name was typically capitalized for several years after its coining.

It has been frequently cited as a major example of a "psychological link between atomic destruction and sexuality" in popular culture, which includes the stenciling of Rita Hayworth onto one of the bombs detonated at Crossroads, and its persistence in language has been argued as having "trivialized and downplayed the reality of nuclear testing," given the contamination done by especially later US thermonuclear tests at Bikini and other Marshallese atolls.

By making an analogy with words like bilingual and bilateral containing the Latin prefix "bi-" (meaning "two" in Latin), the word bikini was first back-derived as consisting of two parts, [bi + kini] by Rudi Gernreich, who introduced the monokini in 1964. Later swimsuit designs like the tankini and trikini further cemented this derivation. Over time the "–kini family" (as dubbed by author William Safire), including the "–ini sisters" (as dubbed by designer Anne Cole), expanded into a variety of swimwear including the monokini (also known as a numokini or unikini), seekini, tankini, camikini, hikini (also hipkini), minikini, face-kini, burkini, and microkini. The Language Report, compiled by lexicographer Susie Dent and published by the Oxford University Press (OUP) in 2003, considers lexicographic inventions like bandeaukini and camkini, two variants of the tankini, important to observe. Although "bikini" was originally a registered trademark of Réard, it has since become genericized.