fill-in vs reliever

fill-in and reliever both are nouns.

Word	Noun	Adjective	Verb	Adverb
fill-in	Yes	No	No	No
reliever	Yes	No	No	No

As nouns, fill-in and reliever are synonyms defined as:

fill-in and reliever: someone who takes the place of another (as when things get dangerous or difficult)

Other synonyms of fill-in include backup, backup man, relief, stand-in, substitute.

fill-in (noun)	reliever (noun)
someone who takes the place of another (as when things get dangerous or difficult)	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