fireman vs fire chief

fireman and fire chief both are nouns.

Word	Noun	Adjective	Verb	Adverb
fireman	Yes	No	No	No
fire chief	Yes	No	No	No

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

Other hyponyms of fireman include fire marshal, fire warden, forest fire fighter, ranger.

fireman (noun)	fire chief (noun)
play in which children pretend to put out a fire	the head of a fire department
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