fireman vs reliever

fireman and reliever both are nouns.

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

As nouns, fireman and reliever are synonyms defined as:

Other synonyms of fireman include relief pitcher.

fireman (noun)	reliever (noun)
play in which children pretend to put out a fire	a pitcher who does not start the game
a member of a fire department who tries to extinguish fires	someone who takes the place of another (as when things get dangerous or difficult)
a laborer who tends fires (as on a coal-fired train or steamship)	a person who reduces the intensity (e.g., of fears) and calms and pacifies
a pitcher who does not start the game