A bikini is a two-piece swimsuit primarily worn by women that features two triangles of fabric on top that cover the breasts, and two triangles of fabric on the bottom: the front covering the pelvis but exposing the navel, and the back covering the intergluteal cleft and often 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.
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, bikini, skirtini, thong, and g-string. A man's single-piece brief swimsuit may also be called a bikini. 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 an US$811 million business annually and boosted spin-off services such as bikini waxing and sun tanning.