An obscenity is a dirty word or phrase. It can also refer to the quality of being lewd, bawdy, or just plain offensive.
You know those four-letter words that everyone knows but not everyone likes to hear? The ones that will get you in trouble if you say them in school or on television? They are obscenities, which are also known as swear words. You can also say that anything offensive or inappropriate is an obscenity. This can include things that are not only dirty, but things that are offensive in other ways. Many say torture and war are obscenities.