A Rash Guard (sometimes called a Rash Vest, Rashie or Swim Shirt) is a type of athletic shirt worn for extra function and protection when surfing, snorkeling, jet skiing, or doing other watersports where you want more protection than a swimsuit, but don't really need a full wetsuit. Most rash guards have long sleeves and while some women's tops show midriff, most are regular shirt length. Of note, they are sometimes worn under a wetsuit or diving suit for added comfort.
Because rash guards are made from tight-fitting materials like spandex, nylon, polyester, or neoprene and are often sold as matching top/bottom sets, this can sometimes make them appear similar to a wetsuit or one-piece swimsuit at a glance when no midriff is visible. A clue to watch for is the horizontal line where the top-piece and bottom-piece come together/overlap. If there is no line clearly identifying top from bottom, assume the garment is a wetsuit instead.