A device for suddenly producing an explosive neutron chain reaction in a fissile material such as uranium-235 (²³⁵U) or plutonium-239 (²³⁹Pu). In a wider sense, any explosive device that derives its energy from nuclear reactions, including not only the foregoing fission weapon but also a fusion weapon, which gets its energy largely from fusion reactions of heavy hydrogen isotopes, and a fission-fusion weapon, which derives its energy from both fission and fusion. Because an atomic bomb derives its energy from nuclear reactions, it is properly called a nuclear explosive or nuclear weapon.