A nuclear weapon is an explosive device that derives its destructive power from nuclear reactions, either fission (as in the case of atomic bombs) or from a combination of fission and fusion reactions (thermonuclear weapons, such as hydrogen bombs). Nuclear weapons have the potential to cause catastrophic damage and casualties on a massive scale, and their indiscriminate use is a major security concern for nations around the world. For this reason, beyond their obvious utility as a deterrent, their development and proliferation are considered a critical issue of international security.

See also: mutually assured destruction, arms race, collective action, catastrophe weapon, existential threat