megalomania vs mental disease

megalomania and mental disease both are nouns.

Word	Noun	Adjective	Verb	Adverb
megalomania	Yes	No	No	No
mental disease	Yes	No	No	No

As nouns, mental disease is a hypernym of megalomania; that is, mental disease is a word with a broader meaning than megalomania:

megalomania: a psychological state characterized by delusions of grandeur
mental disease: any disease of the mind; the psychological state of someone who has emotional or behavioral problems serious enough to require psychiatric intervention

Other hypernyms of megalomania include mental illness, psychopathy.

megalomania (noun)	mental disease (noun)
a psychological state characterized by delusions of grandeur	any disease of the mind; the psychological state of someone who has emotional or behavioral problems serious enough to require psychiatric intervention