A "swimming costume" is another way to say "swimsuit" or "bathing suit." It refers to the clothing that people wear for swimming or being in water. Full definition
You can easily found them and even extra several swimming costumes like that. (promoocodes.com)
I wasn't even allowed to swim because of my recent tattoo but my new swimming costume literally arrived in the mail the day before! (sugarandcinnamon.com)
The idea of a good swimming costume is that it should draw attention to the most flattering parts of your body so you don't look great but also feel comfortable. (myntra.com)