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 Tank. It is often worn in hot weather or while swimming. The shapes of both parts of a suit 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 suit 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 suit, which shocked when it appeared on French beaches in 1947, dates back to antiquity, in Çatalhöyük and the Greco-Roman world. The modern suit was invented by French engineer Louis Réard in 1946. He named it after the suit 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 suit variant, is a back formation from the suit, interpreting the first syllable as the Latin prefix bi- meaning "two" or "doubled", and substituting for it mono- meaning "one". Jacques Heim called his suit precursor the Atome, named for its size, and Louis Réard claimed to have "split the Atome" to make it smaller.