What is the difference between healthy narcissism?
Healthy narcissism is when you love yourself for who you really are. Pathological narcissim is when you are in love with an image of yourself you cannot obtain. Everyone has traits from both, especially in western society where focus is on the individual.
Hmmm... 'Megalomania' is a word derived from Greek. 'Narcissism' is a word derived from the name of a Greek god. Other than that, probably not much... Megalomania is a thirst for fame, glory, spectacle, conquest, etc., and it requires supporting players and an audience. Narcissism is simply falling in love with oneself. Someone could be a narcissist while deprived of human company, but a megalomaniac would be out of business.