A keiretsu is a set of companies with interlocking business relationships and shareholdings that have dominated the Japanese economy since the second half of the 20th century. In the legal sense, it is a type of informal business group that is loosely in an organized alliance within the social world of Japan's business community. It rose up to replace the zaibatsu system that was dissolved in the occupation of Japan following the Second World War. Though their influence has shrunk since the late 20th century, they continue to be important forces in Japan's economy in the early 21st century.

Zaibatsu is a Japanese term referring to industrial and financial vertically integrated business conglomerates in the Empire of Japan, whose influence and size allowed control over significant parts of the Japanese economy from the Meiji period to World War II. A zaibatsu's general structure included a family-owned holding company on top, and a bank which financed the other, mostly industrial subsidiaries within them. Although the zaibatsu played an important role in the Japanese economy beginning in 1868, they especially increased in number and importance following the Russo-Japanese War, World War I, and Japan's subsequent attempt to conquer East Asia and the Pacific Rim during the inter-war period and World War II. After World War II, they were dissolved by the Allied occupation forces and succeeded by the keiretsu. Equivalents to the zaibatsu can still be found in other countries, such as the chaebol conglomerates of South Korea.