Insanity is a legal term that means a serious mental condition that prevents someone from understanding what they are doing or knowing that it is wrong. In court, a person judged insane might not be held fully responsible for their actions because their mind was not working properly when they acted.

The word also gets used informally, though often carelessly, to describe behavior that seems wildly irrational or senseless. When someone says “That's insanity!” about a friend's plan to build a treehouse without any tools, they mean it seems absurdly unrealistic, not that the person is actually mentally ill.

You'll sometimes hear the expression “the definition of insanity is doing the same thing over and over and expecting different results.” This popular saying (often wrongly attributed to Albert Einstein) captures how frustrating it feels to watch someone repeat a failed approach without changing anything. If you keep studying the same wrong way and failing tests, or keep trying to force puzzle pieces together that clearly don't fit, someone might point out you're caught in this cycle.

The noun form is insanity, while insane serves as the adjective.