Organized crime is a category of transnational, national, or local groupings of highly centralized enterprises run by criminals to engage in illegal activity, most commonly for profit. While organized crime is generally thought of as a form of illegal business, some criminal organizations, such as terrorist groups, rebel forces, and separatists, are politically motivated. Sometimes criminal organizations force people to do business with them, such as when a gang extorts money from shopkeepers for "protection". Street gangs may often be deemed organized crime groups or, under stricter definitions of organized crime, may become disciplined enough to be considered organized. A criminal organization can also be referred to as a gang, mafia, mob, ring, or syndicate; the network, subculture, and community of criminals involved in organized crime may be referred to as the underworld. Sociologists sometimes specifically define a "mafia" as a type of organized crime group that specializes in the supply of extra-legal protection and quasi-law enforcement. Academic studies of the original "Mafia", the Sicilian Mafia, which predates the other groups, generated an economic study of organized crime groups and exerted great influence on studies of the Russian mafia, the Chinese Triads, the Hong Kong Triads, and the Japanese Yakuza.