Sin City is a nickname that may be applied to 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.

LAS VEGAS (KLAS)  Locals hate the title, but it's stuck for years and years  Sin City.  What most don't know is the history behind Las Vegas' unshakable nickname. To many, especially tourists and outsiders, the reference is obvious.