General term for swimwear; used for both men's and women's suits. Slightly more formal or written than 水着, but interchangeable in most contexts.
I went to the department store to buy a new swimsuit.
I changed into my swimsuit and went into the ocean.
Compound of 海水 (かいすい, 'seawater') and 着 (ぎ, 'clothing'). The exact historical derivation is uncertain, but the word literally means 'seawater wear'.