critic vs niggler

critic and niggler both are nouns.

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

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

critic: someone who frequently finds fault or makes harsh and unfair judgments
niggler: someone who constantly criticizes in a petty way

Other hyponyms of critic include carper, nitpicker, roaster.

critic (noun)	niggler (noun)
a person who is professionally engaged in the analysis and interpretation of works of art	someone who constantly criticizes in a petty way
anyone who expresses a reasoned judgment of something
someone who frequently finds fault or makes harsh and unfair judgments