overzealous vs fanatic

overzealous is not a noun while fanatic is a noun.

Word	Noun	Adjective	Verb	Adverb
overzealous	No	Yes	No	No
fanatic	Yes	Yes	No	No

As adjectives, overzealous and fanatic are synonyms defined as:

overzealous and fanatic: marked by excessive enthusiasm for and intense devotion to a cause or idea

Other synonyms of overzealous include fanatical, rabid.

overzealous (noun)	fanatic (noun)
	a person motivated by irrational enthusiasm (as for a cause)

overzealous (adjective)	fanatic (adjective)
marked by excessive enthusiasm for and intense devotion to a cause or idea	marked by excessive enthusiasm for and intense devotion to a cause or idea