Delusion
A delusion was a psychological disorder that displayed a false belief that was not necessarily visible to the believer (unlike a hallucination), often brought on by mental illness.

In 2258 of the alternate reality, Doctor  referred to  as being delusional due to a Melvaran mud flea vaccine and took responsibility for his condition. ()

In 2268, Korax accused James T. Kirk of having "delusions of godhood." ()

In 2269, while Kirk was trapped in Janice Lester's body, Doctor Arthur Coleman diagnosed Lester with the delusion of being Captain Kirk. ()

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Medical conditions