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 delusional 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")