reliever vs fill-in

reliever and fill-in both are nouns.

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

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

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

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

reliever (noun)	fill-in (noun)
a pitcher who does not start the game	someone who takes the place of another (as when things get dangerous or difficult)
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