A bikini is a women's 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 a little, 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", while those covering about three-quarters of the buttocks may be described as "cheeky" or "cheeky-cut".

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. Clothing designer Louis Réard introduced his new, smaller design in July. He named the swimsuit after the Bikini Atoll, where the first public test of a nuclear bomb had taken place four days before. His skimpy design was risqué, exposing the wearer's navel and much of her buttocks. No runway model would wear it, so he hired a nude dancer from the Casino de Paris named Micheline Bernardini to model it at a review of swimsuit fashions.

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; Germany banned the bikini from public swimming pools until the 1970s, 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 later 20th century, albeit discreetly.

The bikini gained increased exposure and acceptance as film stars like Brigitte Bardot, Raquel Welch, and Ursula Andress wore them 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.