From AMS Glossary
The eventual descent to the earth's surface of radioactive matter placed in the atmosphere by an atomic or thermonuclear explosion.
Fallout is commonly separated into three classes. 1) Local or close-in fallout occurs in the vicinity (mostly downwind) of the detonation in a matter of hours. It consists mainly of particles greater than 25 microns in diameter, mostly earthen matter rendered radioactive by attachment. Local fallout occurs for an estimated 20%–80% of the total fission products (depending on location and type of detonation). 2) Intermediate or tropospheric fallout remains in the atmosphere for weeks to months, falling out principally by precipitation scavenging, but also by gravitational settling and impingement upon vegetation. Only a few percent of total fission products fall out in this manner. 3) Delayed or stratospheric fallout results from high-yield detonations that thrust their clouds into the stratosphere. Storage times vary from less than one year to as high as ten years, depending upon latitude and altitude of detonation. The particles are of submicron size.