The term radioactive fallout, or just fallout, refers to the debris and radioactive materials that settle out of the air after the detonation of a nuclear weapon or after a nuclear accident that produces a cloud of airborne material, or plume. Detonation of a nuclear weapon results in the immediate propagation of a shock wave and intense heat. As the superheated fireball rises, a vacuum is formed that draws in scorched building material, soil, and other materials from the epicenter of the blast. In addition, radionuclides produced in the nuclear chain reaction leading to the explosion and any weapon material not consumed in that reaction will also be a part of the subsequent plume. Any similar thermal process, such as the intense fire during the Chernobyl I reactor accident in 1986, will introduce radioactive and other materials into the atmosphere, as well.

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