The piece of clothing you wear to swim in. In everyday use it usually means a woman's or girl's one, and most often the one-piece kind that covers the body — that is the picture most people get first. For men, English speakers usually say swimming trunks, swim shorts, or just trunks instead. There are lots of regional words here, which is why students get confused: British speakers often say swimming costume or cossie, older speakers might say bathing suit (still very common in America), and 'bikini' is the specific two-piece. 'Swimsuit' is the safest, most neutral word of the lot, understood everywhere. Note that it is one word, no space, and it is countable: a swimsuit, two swimsuits. In Taiwan you will meet it on shop signs and at every pool, often next to the rule that a swimming cap is required too.