psychosis vs mental disease

psychosis and mental disease both are nouns.

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

As nouns, mental disease is a hypernym of psychosis; that is, mental disease 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 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 psychosis include mental illness, psychopathy.

psychosis (noun)	mental disease (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