critic vs nitpicker

critic and nitpicker both are nouns.

Word	Noun	Adjective	Verb	Adverb
critic	Yes	No	No	No
nitpicker	Yes	No	No	No

As nouns, nitpicker is a hyponym of critic; that is, nitpicker is a word with a more specific, narrower meaning than critic:

critic: someone who frequently finds fault or makes harsh and unfair judgments
nitpicker: someone who makes small and unjustified criticisms

Other hyponyms of critic include carper, niggler, roaster.

critic (noun)	nitpicker (noun)
a person who is professionally engaged in the analysis and interpretation of works of art	someone who makes small and unjustified criticisms
anyone who expresses a reasoned judgment of something
someone who frequently finds fault or makes harsh and unfair judgments