A bikini or two piece is a women's swimsuit with two parts, one covering the breasts (optionally in the case of the monokini), the other the groin (and optionally the buttocks), leaving an uncovered area between the two (optionally in the case of the Tankini). It is often worn in hot weather or while swimming.

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.