psychosis vs mental illness

psychosis and mental illness both are nouns.

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

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

psychosis: any severe mental disorder in which contact with reality is lost or highly distorted
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 psychosis include mental disease, psychopathy.

psychosis (noun)	mental illness (noun)
any severe mental disorder in which contact with reality is lost or highly distorted	any disease of the mind; the psychological state of someone who has emotional or behavioral problems serious enough to require psychiatric intervention