The term “Vietminh” (or Viet Minh) was an abbreviation for the much longer name of a group that fought for the independence of Vietnam beginning in 1941. This group was both nationalist and communist. There is disagreement as to whether it was more of a communist group or more of a nationalist one. The Vietminh was led by Ho Chi Minh.
Before World War II, Vietnam had been a French colony. Ho and many other Vietnamese wanted independence from France. In 1941, Japan occupied Vietnam as France had lost the war in Europe to Japan’s ally, Germany. The Vietminh then turned to resisting the Japanese. It eventually received help from the Americans in this endeavor.
When WWII ended, the Vietminh hoped for independence. However, the US wanted to allow France to renew ownership of Vietnam. This led to a war between the Vietminh and the French. The Vietminh eventually won the war and Vietnam was split into North Vietnam and South Vietnam. At that point, the Vietminh essentially became the government of North Vietnam and ceased to exist as a group.