insanity vs mental illness

insanity and mental illness both are nouns.

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

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

insanity: relatively permanent disorder of the mind
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 insanity include mental disease, psychopathy.

insanity (noun)	mental illness (noun)
relatively permanent disorder of the mind	any disease of the mind; the psychological state of someone who has emotional or behavioral problems serious enough to require psychiatric intervention