Idiocy means extreme foolishness or stupidity. When someone does something incredibly unwise or senseless, you might call it an act of idiocy. If a student who studied hard all semester decides to skip the final exam to play video games, that would be pure idiocy. If someone tries to dry their phone in a microwave after dropping it in water, that's an example of idiocy.

The word suggests doing something so obviously foolish that almost anyone could see it was a terrible idea. It's the difference between taking a wrong turn because you misread a map (an honest mistake) and refusing to look at the map at all even though you're completely lost (idiocy).

Sometimes people use this word too harshly for minor mistakes. It's also a word that can feel insulting when it's aimed at a person instead of an action. But in its strongest sense, idiocy means ignoring common sense, refusing to learn from obvious facts, or making choices that no reasonable person would make. When a country's leaders make a disastrous decision despite clear warnings from experts, historians might later call it an act of collective idiocy.

The related adjective is idiotic, as in “That was an idiotic thing to do.”