Noun

1. The event of small airborne particles falling to the ground in significant quantities as a result of major industrial activity, volcano eruption, sandstorm, nuclear explosion, etc.
2. The particles themselves.

   On 26 April 1986 the reactor number four at the Chernobyl Nuclear Power Plant located in the Soviet Union near Pripyat in Ukraine exploded. Further explosions and the resulting fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical area.

3. A negative side effect; an undesirable or unexpected consequence.

   Psychological fallout in the shadow of terrorism, title of an article by Dr. Abraham Twerski, M.D. in http://www.jewishworldreview.com.

4. A declined offer in a sales transaction when acceptance was presumed.
5. The person who declines such an offer.