The bikini or two piece is a women's swimsuit with two parts, one covering the breasts, the other the groin and, optionally, part or all of the buttocks, leaving an uncovered area between the two. It is often worn in hot weather, while swimming or sunning. 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".

While two-piece bathing suits had been worn on the beach before, 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.