The Sicilian Mafia or Cosa Nostra (Italian: [ˈkɔːza ˈnɔstra, ˈkɔːsa -], Sicilian: [ˈkɔːsa ˈnɔʂː(ɽ)a]; "our thing"), also referred to as simply Mafia, is a criminal society originating on the island of Sicily and dates back to the mid-19th century. It is an association of gangs which sell their protection and arbitration services under a common brand. The Mafia's core activities are protection racketeering, the arbitration of disputes between criminals, and the organizing and oversight of illegal agreements and transactions.

The basic group is known as a "family", "clan", or cosca. Each family claims sovereignty over a territory, usually a town, village or neighborhood (borgata) of a larger city, in which it operates its rackets. Its members call themselves "men of honor", although the public often refers to them as mafiosi. By the 20th century, wide-scale emigration from Sicily led to the formation of mafiosi style gangs in Australia, the United Kingdom, Canada, the United States and South America. These diaspora-based outfits replicated the traditions and methods of their Sicilian ancestors to varying extents.