hal ‧ lu ‧ ci ‧ na ‧ tion /həˌluːsəˈneɪʃ ə n, həˌluːsɪˈneɪʃ ə n/ BrE AmE noun
[uncountable and countable] something which you imagine you can see or hear, but which is not really there, or the experience of this:
The patients suffered hallucinations caused by the drug.