Fallout is the cloud of radioactive material that falls from the sky after a nuclear blast. Fear of the effects of fallout is just one reason some people are nervous about nuclear energy.
Nuclear fallout settles on the ground and in the atmosphere after a nuclear bomb explodes or a reaction occurs at a damaged nuclear power plant. The dangers of this kind of fallout are enormous, including immediate death and long-term illness caused by breathing and eating the radioactive dust. Another kind of fallout is more figurative — it's any kind of negative effect or result, like the fallout from telling your family you're getting an enormous tattoo.