fanatic vs overzealous

fanatic is a noun but overzealous is not a noun.

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

As adjectives, fanatic and overzealous are synonyms defined as:

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

Other synonyms of fanatic include fanatical, rabid.

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

fanatic (adjective)	overzealous (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