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Notionally, the cause of bit rot. However, this is asemi-independent usage that may be invoked as a humorous wayto handwave away any minor randomness that doesn't seemworth the bother of investigating. "Hey, Eric - I just gota burst of garbage on my tube, where did that come from?""Cosmic rays, I guess." Compare sunspots, phase of themoon. The British seem to prefer the usage "cosmic showers";"alpha particles" is also heard, because stray alpha particlespassing through a memory chip can cause single bit errors(this becomes increasingly more likely as memory sizes anddensities increase).Factual note: Alpha particles cause bit rot, cosmic rays donot (except occasionally in spaceborne computers). Intelcould not explain random bit drops in their early chips, andone hypothesis was cosmic rays. So they created the World'sLargest Lead Safe, using 25 tons of the stuff, and used twoidentical boards for testing. One was placed in the safe, oneoutside. The hypothesis was that if cosmic rays were causingthe bit drops, they should see a statistically significantdifference between the error rates on the two boards. Theydid not observe such a difference. Further investigationdemonstrated conclusively that the bit drops were due to alphaparticle emissions from thorium (and to a much lesser degreeuranium) in the encapsulation material. Since it isimpossible to eliminate these radioactives (they are uniformlydistributed through the earth's crust, with the statisticallyinsignificant exception of uranium lodes) it became obviousthat one has to design memories to withstand these hits.