Clothing for swimming, spoken about as a general category rather than as one item. This is the important difference from 'swimsuit': swimwear is an uncountable word, so you say 'a shop that sells swimwear', never 'a swimwear' or 'two swimwears'. It covers everything in that department — women's one-pieces, bikinis, men's trunks, children's suits — all together. Because of that, it is the word of shops, labels, catalogues, and adverts: the swimwear section, the summer swimwear range. Ordinary people talking to each other rarely say it; a friend says 'bring your swimsuit', not 'bring your swimwear'. So the word feels commercial and slightly formal, and if you use it in casual conversation you will sound like a shop sign. English has a whole family built this way — footwear, sportswear, menswear, knitwear — and they all behave the same: category words, uncountable, at home in shops.