Demented means having lost the ability to think clearly and rationally, usually because of illness or brain disease. When doctors describe someone as demented, they mean that person's mind no longer works the way it once did. They might forget important things, become confused about where they are, or struggle to recognize people they've known for years.

The word comes from a serious medical condition called dementia, which often affects elderly people. Someone with dementia might forget conversations they just had, get lost in familiar places, or have trouble doing tasks they once did easily. Alzheimer's disease is one type of dementia that causes these problems.

People sometimes use demented more loosely to describe behavior that seems wildly irrational, like calling a villain's plan in a movie “demented.” But this casual use can be insensitive, since the word originally describes a real condition that affects millions of families.