/ həˌluːsɪˈneɪʃn; NAmE / noun
1.
[ C , U ] the fact of seeming to see or hear sb/sth that is not really there, especially because of illness or drugs :
to have hallucinations
High temperatures can cause hallucination.
2.
[ C ] something that is seen or heard when it is not really there :
Was the figure real or just a hallucination?
HELP NOTE : Some speakers do not pronounce the 'h' at the beginning of hallucination and use 'an' instead of 'a' before it. This now sounds old-fashioned.
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WORD ORIGIN
early 17th cent.: from Latin hallucinatio(n-) , from the verb hallucinari , from Greek alussein be uneasy or distraught.