A shameless and impractical ensemble of clothes that do not leave much to the imagination when worn. With some designs, the wearer may as well be nude for all the cover they provide.
This doesn't include lingerie, swimsuits, bikini armor or similar pieces where the revealing elements are largely expected out of such articles (such as underwear, harem outfits, or certain cultural outfits). Likewise, torn clothes, wet clothes, dissolving clothes or other external events that cause partial nudity don't count either because the revealing elements are accidental.
Revealing clothes are intentionally (and somewhat hilariously) designed to be showy and not for real-world functionality - they only exist to show off as much of the body as possible while utilizing minimal coverage. Several tags involved here include see-through clothes, miniskirt (or the even smaller microskirt), naked suspenders, crotch plate, thong, bikini top, cleavage, side- and/or underboob, and a distinct lack of bras and panties. Thighhighs are also popular to show off the upper legs by utilizing zettai ryouiki.