Vulva is the correct name for the outer parts of a woman's sexual organs — the parts you could see from outside, rather than anything inside the body. It is the everyday-but-accurate word used by doctors, nurses, biology teachers, health websites and sex-education books. It is not rude and it is not slang; it is simply the right term, in the same way 'nostril' is the right term for part of a nose. Because English has few polite words in this area, 'vulva' does a lot of work: when a health worker or a writer wants to be clear and respectful, this is the word they reach for. Grammatically it is an ordinary countable noun — the vulva, her vulva. The plural is usually 'vulvas'; you may also see the Latin plural 'vulvae' in medical writing. Two related words often appear beside it: 'vaginal' and 'vulval' (or 'vulvar'), which are their adjectives. In real use there is a great deal of confusion, even among native speakers, between this word and 'vagina', which is why health campaigns keep explaining the difference. Culturally, English speakers vary a lot in how comfortable they are saying it out loud; in a clinic or a classroom it is completely normal, while in casual conversation many people avoid the whole subject. As a learner you are safest using it exactly where a doctor would.