A hallucination is when someone sees, hears, or senses something that isn't actually there. A person experiencing a hallucination might see a dog in their room when there's no dog, hear voices when nobody's speaking, or feel bugs crawling on their skin when nothing's there. To them, these sensations feel completely real, even though other people can't perceive them.

Hallucinations can happen for several reasons. High fevers sometimes cause them, which is why a very sick child might see strange things until their fever breaks. Certain medical conditions affect the brain in ways that produce hallucinations. Extreme lack of sleep can also trigger them: if you stay awake for days, your exhausted brain might start creating sensory experiences that aren't real.

In technology, people now use hallucination to describe when artificial intelligence systems confidently present made-up information as fact. When an AI hallucinates, it might invent book titles that don't exist or create fake historical events, presenting these fictions as real information. Just like a person experiencing a hallucination perceives things that aren't there, an AI that hallucinates generates content that seems real but has no basis in actual evidence or truth.