defender vs fireman

defender and fireman both are nouns.

Word	Noun	Adjective	Verb	Adverb
defender	Yes	No	No	No
fireman	Yes	No	No	No

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

defender (noun)	fireman (noun)
a person who cares for persons or property	play in which children pretend to put out a fire
a fighter who holds out against attack	a member of a fire department who tries to extinguish fires
	a laborer who tends fires (as on a coal-fired train or steamship)
	a pitcher who does not start the game