Ultraviolet, usually shortened to UV, is light your eyes cannot see. Picture a rainbow: red at one end, violet at the other. Ultraviolet is the light just past the violet end — still light, still travelling from the sun, but your eyes have no way to catch it. It is an adjective (ultraviolet light, ultraviolet rays, ultraviolet radiation) and also a noun for the light itself ("protects you from ultraviolet"). In real speech people almost always say "UV" — UV index, UV protection, UV rays — and "ultraviolet" written out sounds more like a textbook or a label. UV is the part of sunlight that changes your skin: it tans you, burns you if you get too much, and over years it wrinkles and damages skin and can cause cancer. That is what sunscreen blocks, and why the weather report here gives a UV level even on a cool day — the temperature and the UV strength are two different things, which is why you can burn badly on a windy spring afternoon. UV is also used on purpose: it kills germs, so it is used in hospitals and water purifiers; it makes some things glow, which is how banknotes are checked and how a "black light" works; and it hardens gel nail polish. One warning worth remembering: UV goes through cloud and through water, so a grey day and a swimming pool both give less protection than they feel like they do.