A bikini or two-piece is a type of women's bathing suit, characterized by two separate parts—one covering the breasts, the other the groin (and optionally the buttocks), leaving an uncovered area between the two garments. The shapes of both parts of a bikini closely resemble women's underwear, and the lower part of a bikini can therefore range from the more revealing thong or g-string to briefs and the more modest square-cut shorts.

For information about the bikini please visit Wikipedia. Note: the above defination was from Wikipedia as well.