boxer vs welterweight

boxer and welterweight both are nouns.

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

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

boxer (noun)	welterweight (noun)
a breed of stocky medium-sized short-haired dog with a brindled coat and square-jawed muzzle developed in Germany	A weight class division in combat sports, for fighters heavier than those in the lightweight division and lighter than those in the middleweight division.
someone who fights with his fists for sport	a professional boxer who weighs between 141 and 147 pounds
a workman employed to pack things into containers	a wrestler who weighs 154-172 pounds
	an amateur boxer who weighs no more than 148 pounds
	a weight of 28 pounds; sometimes imposed as a handicap in a horse race (such as a steeplechase)