Mitsubishi Zaibatsu
Refers to the pre-WWII Mitsubishi conglomerate, one of the major family-controlled industrial and financial combines in Japan.
The Mitsubishi Zaibatsu was one of the four major conglomerates that supported the pre-war Japanese economy.
三菱グループ is the modern, loosely affiliated corporate group that succeeded the pre-war 三菱財閥 after the dissolution of zaibatsu.
財閥 is the general term for a family-controlled conglomerate; 三菱財閥 is a specific historical example.
Compound of 三菱 (Mitsubishi, the family/company name) and 財閥 (zaibatsu, financial clique). The Mitsubishi name originates from the combination of the three-diamond crest (三つ菱) of the Iwasaki family and the Yamauchi family.