Fallout is when a powder eyeshadow drops onto one's face and collects on the cheeks or under the eyes. Similar to nuclear fallout, it is devastating. Some eyeshadows are powdery and tend to drop from the brush during application while some don't. This is due to something something chemistry. Fallout is a problem because if you've already done your face makeup (foundation, powder, blush), then getting dark eyeshadow on your cheeks will ruin your makeup. See below:
There are several ways to minimize fallout. First, you can spray your brush prior to picking up a shadow so that it sticks to the brush more and then it'll stick to your eye. Another way, my preferred way, is to just apply your face makeup last. Then if there's fallout, you just wipe it away and theeen do your face makeup. You can also purchase face shields that have a piece of adhesive on one end so they hang off your face and collect any fallout. I'm just not into the idea of spending money on something that has a simple solution. I have seen some "diy" examples of these using things like panty liners. But still...too much work. Finally, there is a technique called baking which is when you place a layer of face powder under the eyes and when there's fallout, it lands on the powder. Then you take a brush and dust it all off. I've never tried baking and I'm not really interested in trying it only because I don't like the idea of using a lot of powder under my eyes especially now that I'm starting to get some fine lines there. Powder tends to emphasize that kind of stuff. Sometimes a little bit of fallout is inevitable as is the case with glitter. I've found that glittery eyeshadows tend to just magically float off your eyelid and land all over the face. It's in glitter's cold nature to betray you.