To hallucinate means to see, hear, or sense something that isn't really there. When someone hallucinates, their brain creates experiences that feel completely real but don't exist in the physical world. A person with a high fever might hallucinate and think they see spiders crawling on the walls when the walls are actually empty. Someone lost in the desert might hallucinate an oasis that isn't there.

Hallucinations are different from dreams or imagination. When you dream, you know you're asleep. When you imagine something, you understand it's in your mind. But when someone hallucinates, they truly believe what they're experiencing is real, at least in that moment. Hallucinations can happen because of illness, certain medical conditions, or as side effects of some medications.

The word has recently taken on a new meaning in technology. When artificial intelligence systems hallucinate, they confidently provide information that sounds true but is completely made up. An AI might hallucinate facts about history, invent book titles that don't exist, or create realistic-sounding details that have no basis in reality. Just as a person hallucinating senses things that aren't there, an AI that hallucinates generates content that seems real but isn't actually true.