fireman vs defender

fireman and defender both are nouns.

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

As nouns, defender is a hypernym of fireman; that is, defender is a word with a broader meaning than fireman:

Other hypernyms of fireman include guardian, protector, shielder.

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