Slang(俚语) is one of those things that everybody can recognize and nobody can define(下定义). Not only is it hard to wrap slang in a definition, it is also hard to distinguish (区别)it from such similar things as colloquialism (口语) , provincialism (方言) , jargon (行话) or trade talk (行业话). Usually, slang tends to be transient (短暂的). Thus their children often laugh at parents when this older generation uses slang which was considered to be the height of fashion in their own youth. Of course, the slang teenage children use today is very different from that of their parents. Indeed it might ever be some obscure (晦涩的) foreign language as far as the older generation is concerned for it is totally beyond their understanding. It is often said that a slang term stops to be slang when it is 'accepted by the dictionary'. This is not really the ease. You will find many slang terms listed in dictionaries are still slang terms. The term stops to be slang when it drives (派生) its respectable synonym (同义词)out of use, or when it gets a meaning that cannot be expressed otherwise. Such things have happened. The term 'hot dog' was once a slang term, but it couldn't be considered so now. No one in America would go up to a counter and order 'a sausage sandwich'. Similarly 'vanity', originally a slang contraction (缩写) of university, has acquired a special meaning which only it expresses and is no longer slang. Jazz, when it means a particular kind of music, is scarcely a slang form, since there is no more respectable word meaning that kind of music. What can readers know about slang from this article?