(covers information from several alternate timelines)
Megalomania is a mental condition describing someone who seeks power and grandeur.
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")