Insane means having a severe mental illness that makes it very hard or impossible for someone to understand reality or control their behavior. In medical and legal contexts, the word describes a serious condition where someone's mind isn't working properly, so they may not understand what is real or what is happening around them.

In everyday conversation, people often use insane much more loosely to mean extremely foolish, wildly unreasonable, or shockingly extreme. Someone might call an idea insane if it seems completely impractical: “It would be insane to try biking 100 miles without any training.” A person might describe a roller coaster as insane because of its terrifying loops and drops. These casual uses really just mean “extremely” or “unbelievably,” but they're not talking about actual mental illness.

Because the word can sound harsh or disrespectful when referring to people with mental health challenges, many people prefer more precise terms like “mental illness” or specific condition names. In courts of law, there is an “insanity defense,” where lawyers argue that a defendant was too mentally ill to understand their actions or what was happening at the time. This legal idea recognizes that someone whose mind isn't working properly may not be responsible in the same way as other people are.