(covers information from several alternate timelines)
Megalomania was a mental condition characterized by the patient's overly high opinion of themselves and a need to be praised.
In 2374 in the alternate timeline known as the "Year of Hell," Lieutenant Tom Paris stated to Commander Chakotay regarding Captain Annorax's belief that time had a personal grudge against him, "That's called paranoia, Chakotay, with a hint of megalomania." (VOY: "Year of Hell, Part II")
In 2375, Tom Paris, while explaining to Kathryn Janeway how to treat Doctor Chaotica in the The Adventures of Captain Proton holoprogram, described Chaotica as a megalomaniac and suggested she appeal to his ego. (VOY: "Bride of Chaotica!")
In 2377, The Doctor stated that the hologram Iden had been exhibiting some of the classic signs of megalomania, seeing himself as a spiritual leader who was trying to enlarge his flock. (VOY: "Flesh and Blood")