fallout noun [U] (UNPLEASANT RESULT) the unpleasant results or effects of an action or event:

noun the settling to the ground of airborne particles ejected into the atmosphere from the earth by explosions, eruptions, forest fires, etc., especially such settling from nuclear explosions radioactive fallout.

: the often radioactive particles stirred up by or resulting from a nuclear explosion and descending through the atmosphere also : other polluting particles (such as volcanic ash) descending likewise b : descent (as of fallout) through the atmosphere 2 : a secondary and often lingering effect, result, or set of consequences