reliever vs relief pitcher

reliever and relief pitcher both are nouns.

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

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

Other synonyms of reliever include fireman.

reliever (noun)	relief pitcher (noun)
a pitcher who does not start the game	a pitcher who does not start the game
someone who takes the place of another (as when things get dangerous or difficult)
a person who reduces the intensity (e.g., of fears) and calms and pacifies