The Collaborative International Dictionary of English v.0.48:

Atom bomb \At"om bomb`\, Atomic bomb \A*tom"ic bomb`\n.
   a bomb of great power in which the explosive energy is
   derived from the nuclear fission of a fissionable material,
   such as plutonium or uranium-235. It is a type of nuclear
   weapon. The strength of an atom bomb is measured in kilotons,
   i.e. the equivalent energy released by explosion of a
   thousand tons of TNT. The first atomic bomb was manufactured
   in 1945.

   Syn: A-bomb, fission bomb.
        [PJC] Atomic

WordNet (r) 3.0 (2006):

atom bomb
    n 1: a nuclear weapon in which enormous energy is released by
         nuclear fission (splitting the nuclei of a heavy element
         like uranium 235 or plutonium 239) [syn: atom bomb,
         atomic bomb, A-bomb, fission bomb, plutonium bomb]