“Slang” refers to a wide range of vernacular speech styles, notably the language of the underworld (a major source of innovation in late 20th-21st century speech), of teenagers, and movies and comics.
Anyone who compiles a list of slang is faced continually with the question of « What is slang?”
One possible criterion for defining slang might well be that it cannot be “written down” in standard Chinese, though of course this approach, while useful in some contexts, also begs many questions, not least the definition of “standard Chinese”.
Jonathan Green defines the scope of his dictionary “Landmark Dictionary of Slang (1998)” as follows:
“Slang is the counter-language. A jackanapes lexicon of the dispossessed. The language of the rebel, the outlaw, the despised, the marginal, the young. Above all it is the language of the city – urgent, pointed, witty, cruel, and capable both of excluding and including, of mocking and confirming”.
In addition to “slang”, there are in English a number of partially overlapping terms: “argot”, “cant”, “colloquial”, “dialect”, “demotic speech”, “flash language”, “jargon”, “vernacular”, “swearing”, etc
Green notes that “the line between slang and colloquialism, the casual language of everyday speech, is simply too close to drawn with any facility”.