Stupidity is a lack of good sense or intelligence, or behavior that shows poor judgment. When someone acts with stupidity, they're making foolish choices despite having the ability to think things through. A student shows stupidity by refusing to study for an important test, then complaining about the bad grade. A driver shows stupidity by texting while driving, putting everyone at risk for no good reason.

Stupidity is different from simply not knowing something. If you've never learned how fractions work, that's not stupidity: that's just lack of knowledge. But if your teacher explains fractions carefully, and you refuse to pay attention because you've decided math is boring, that crosses into stupidity. The key difference is that stupidity involves ignoring what you could or should know, or making obviously bad choices.

The word can describe a single foolish act (“What stupidity made him think he could jump off the diving board in the shallow end?”) or someone's general approach to life. A person might do something stupid without being a stupid person overall: everyone makes poor choices sometimes. However, someone who repeatedly ignores obvious facts, refuses to learn from mistakes, or makes the same bad decisions over and over demonstrates real stupidity.

The word can be harsh when directed at a person, so use it carefully. It's often better to call an action or decision stupid rather than the person themselves.