nuclear fallout

Meanings

1. Residual radioactive material in the upper atmosphere which settles down to Earth over a protracted period; typically ash which has been propelled upwards by a nuclear explosion or disaster, sometimes mixed with rain or snow.
2. An event in which this occurs, generally perceived as an ecological disaster due to the distribution of irradiated material over a much larger area than the zone directly affected by the original event.

Etymology

In reference to such material falling out of the sky.