What was the atomic bomb dropped on Nagasaki in 1945 called?

The atomic bomb dropped on Nagasaki on 9 August 1945 was called Fat Man.

Fat Man was a plutonium-239 implosion-type nuclear weapon developed at Los Alamos Laboratory. The B-29 bomber Bockscar dropped it over Nagasaki during the second and most recent wartime use of a nuclear weapon. The bomb detonated at approximately 11:02 a.m. local time.

Its design used conventional explosives arranged around a plutonium core. When the explosives fired simultaneously, they compressed the core and initiated a nuclear chain reaction. This was more technically complex than Little Boy's uranium gun-type mechanism and had been tested in the Trinity nuclear test in New Mexico on 16 July 1945.

Fat Man's name is often confused with Little Boy, but the two weapons differed in shape, fuel, and engineering. Fat Man was broad and rounded, while Little Boy was long and narrow. The Nagasaki bombing, combined with the Hiroshima bombing and the Soviet entry into the war, was followed by Japan's surrender announcement on 15 August.