atom bomb 
(also atomic bomb)
noun
A bomb which derives its destructive power from the rapid release of nuclear energy by fission of heavy atomic nuclei, causing damage through heat, blast, and radioactivity.The simple atom bomb owed its explosive power to the energy released by nuclear fission, or fusion....
- The blast was 600 times as violent as the Hiroshima atom bomb and shot a nuclear cloud more than 17 miles high into the air.
- Now and then, historic images are etched in the mass mind: the awesome mushroom cloud of the atom bomb will forever represent the specter of nuclear destruction.
Such a bomb contains a critical mass of a material such as uranium-235 or plutonium-239, which when detonated by a conventional explosive charge is capable of maintaining a nuclear chain reaction, releasing large amounts of energy almost instantaneously.