Fat Man was the U.S. codename for the plutonium bomb dropped on Nagasaki.
The bomb was dropped on August 9, 1945, by the B-29 bomber Bockscar. Fat Man used a plutonium implosion design, unlike Little Boy, the uranium gun-type bomb dropped on Hiroshima three days earlier. The two weapons therefore differed in both their nuclear material and their internal engineering.
Nagasaki was not the original primary target for the mission. Kokura was selected first, but cloud and smoke obscured the aiming point. The aircraft moved to Nagasaki, where a break in the weather allowed the bomb to be released. The explosion caused enormous destruction and loss of life.
Japan announced its surrender on August 15, six days after the Nagasaki bombing. Fat Man is sometimes confused with Trinity, which was the codename for the first nuclear weapons test in New Mexico on July 16, 1945, rather than a bomb used in combat.