delusion vs mental state

delusion and mental state both are nouns.

Word	Noun	Adjective	Verb	Adverb
delusion	Yes	No	No	No
mental state	Yes	No	No	No

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

delusion: (psychology) an erroneous belief that is held in the face of evidence to the contrary
mental state: (psychology) a mental condition in which the qualities of a state are relatively constant even though the state itself may be dynamic

Other hypernyms of delusion include mental condition, psychological condition, psychological state.

delusion (noun)	mental state (noun)
the act of deluding; deception by creating illusory ideas	(psychology) a mental condition in which the qualities of a state are relatively constant even though the state itself may be dynamic
a mistaken or unfounded opinion or idea
(psychology) an erroneous belief that is held in the face of evidence to the contrary