Beachwear is the clothes people put on for the beach or the pool: a swimsuit, a covering shirt or dress over it, light shorts, sometimes a hat. Shops and packing lists like this word because it names the whole set at once. In ordinary talk you more often name the piece — “swimsuit,” “bikini,” “flip-flops” — than say beachwear. It is usually uncountable: you buy beachwear, not “a beachwear” or “beachwears.” It is not a uniform; it is just “what you wear when you are going to swim or lie in the sun.” Though actually the edges are loose: some people fold sunglasses and a towel into the idea, and some shops put almost any summer clothes under the same sign.