Delirium is a state of severe mental confusion that happens suddenly, usually because of illness, high fever, or injury. When someone experiences delirium, their mind becomes foggy and disorganized. They might not know where they are, what time it is, or even recognize people they know well. They might see or hear things that aren't really there, or say things that don't make sense.

Delirium is different from simply feeling tired or distracted. It's a medical condition that happens when something disrupts the normal functioning of the brain. A child with a dangerously high fever might experience brief delirium, speaking nonsensically or seeming frightened by things that aren't real. Doctors treat the underlying cause, whether it's an infection, dehydration, or another medical problem, and the delirium typically fades as the person recovers.

The word can also describe any state of wild excitement or confusion. A crowd might erupt into delirium when their team wins the championship, or a classroom might dissolve into delirious chaos on the last day before summer vacation. In these cases, delirious means extremely excited or happy, though still somewhat out of control.