A gang is a group or society of associates, friends, or members of a family with a defined leadership and internal organization that identifies with or claims control over territory in a community and engages, either individually or collectively, in illegal, and possibly violent, behavior, with such behavior often constituting a form of organized crime.

Organized crime is a category of transnational, national, or local group of centralized enterprises run 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. Many criminal organizations rely on fear or terror to achieve their goals or aims as well as to maintain control within the organization and may adopt tactics commonly used by authoritarian regimes to maintain power. Some forms of organized crime simply exist to cater towards demand of illegal goods in a state or to facilitate trade of goods and services that may have been banned by a state. Sometimes, criminal organizations force people to do business with them, such as when a gang extorts protection money from shopkeepers. 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 an outfit, a gang, crime family, mafia, mob, (crime) ring, or syndicate; the network, subculture, and community of criminals involved in organized crime may be referred to as the underworld or gangland. Sociologists sometimes specifically distinguish 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 Italian Mafia 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.