fireman vs relief pitcher

fireman and relief pitcher both are nouns.

Word	Noun	Adjective	Verb	Adverb
fireman	Yes	No	No	No
relief pitcher	Yes	No	No	No

As nouns, fireman and relief pitcher are synonyms defined as:

Other synonyms of fireman include reliever.

fireman (noun)	relief pitcher (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
a laborer who tends fires (as on a coal-fired train or steamship)
a pitcher who does not start the game