A megalomaniac is a pathological egotist, that is, someone with a psychological disorder with symptoms like delusions of grandeur and an obsession with power. We also use the word megalomaniac more informally for people who behave as if they're convinced of their absolute power and greatness. Your garden-variety egomaniac might be self-centered and overconfident but he's a little softy compared with the megalomaniac who wants to control the world.