fighter vs boxer

fighter and boxer both are nouns.

Word	Noun	Adjective	Verb	Adverb
fighter	Yes	No	No	No
boxer	Yes	No	No	No

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

fighter (noun)	boxer (noun)
a high-speed military or naval airplane designed to destroy enemy aircraft in the air	a breed of stocky medium-sized short-haired dog with a brindled coat and square-jawed muzzle developed in Germany
someone who fights (or is fighting)	someone who fights with his fists for sport
someone who fights for a cause	a workman employed to pack things into containers