Multiple realities(covers information from several alternate timelines)
A delusion was 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 Leonard McCoy referred to James T. Kirk as being delusional due to a Melvaran mud flea vaccine and took responsibility for his condition. (Star Trek)
In 2268, Korax accused James T. Kirk of having "delusions of godhood." (TOS: "The Trouble with Tribbles")
In 2269, while Kirk was trapped in Janice Lester's body, Doctor Arthur Coleman diagnosed Lester with the delusion of being Captain Kirk. (TOS: "Turnabout Intruder")