An STD is an illness that passes from one person to another during sex. AIDS, herpes, and several others are STDs. The letters stand for sexually transmitted disease. It is a plain medical term, and doctors, nurses, health teachers, and newspapers all use it. It is not rude and it is not slang — but it is a serious topic, so people usually say it carefully, and it is not a word that comes up in light conversation. You will see it on health posters, in clinics, and in school health classes. It is far more common than the full three words; almost nobody says 'sexually transmitted disease' out loud. Say the letters: S-T-D. It takes a/an like a normal noun — 'an STD' — because the letter S is said 'ess', which starts with a vowel sound. There is a newer term you should know: STI, sexually transmitted infection. Doctors increasingly prefer STI, because a person can carry an infection and pass it on while having no symptoms at all — so 'disease' is not quite right for those cases. Both terms are in use; STD is older and still very common, STI is the more careful modern one. Worth knowing: in British English, STD also used to mean something completely unrelated — a long-distance phone call system (subscriber trunk dialling). Older British people may remember 'STD code' for a phone area code. That use has almost disappeared.