megalomania vs mental illness

megalomania and mental illness both are nouns.

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

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

megalomania: a psychological state characterized by delusions of grandeur
mental illness: 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 disease, psychopathy.

megalomania (noun)	mental illness (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