Seeing, hearing, or sensing something that is not really there, usually caused by illness, drugs, or extreme stress.
From Latin 'hallucinari' meaning to wander mentally or dream. The root may relate to Greek 'alussasthai' (to be uneasy). The term entered medical use in the 19th century to describe sensory perceptions without external stimuli.
Hallucinations aren't signs of weakness or insanity—they're what brains do when deprived of normal input (sensory deprivation produces hallucinations in healthy people), showing how dependent our sense of reality is on constant outside confirmation.
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