Relating to or producing hallucinations; involving false or distorted sensory perceptions.
From hallucination with the adjectival suffix -ory (meaning 'relating to'), creating an adjective that describes things connected to hallucinations.
Hallucinatory experiences are so vivid because they bypass normal sensory organs—your brain is creating the entire experience, which is why you can't just 'un-see' a hallucination by closing your eyes like you could with something you actually see.
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