One-Piece Swimsuit. A one-piece swimsuit is a usually a skin-tight women's swimsuits used when swimming in the sea or pool. The one-piece swimsuit usually covers most of the torso and all of the butt. Sporty but sweet, this floral one piece by ZeroXposure will show off her style. Is the bikini going out of fashion? Most U.S. women will be wearing modest one-piece swimsuits this summer (well, if it's good enough for Kim and Sofia).