relief pitcher vs reliever

relief pitcher and reliever both are nouns.

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

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

Other synonyms of relief pitcher include fireman.

relief pitcher (noun)	reliever (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