Sin City is an urban area (a city or part of) that caters to various vices. These vices may be legal (depending on area) or illegal activities which are tolerated.

Examples of vices include sex-related services (prostitution, strip clubs, sex shops, etc.), gambling (casinos, betting shops, etc.), or drug use (alcohol, marijuana, etc. consumption), and even excessive organized crime and gang activity. If the city is known for prostitution, it is often called a red-light district, as in Amsterdam, Netherlands.