fill-in vs replacement

fill-in and replacement both are nouns.

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

As nouns, replacement is a hyponym of fill-in; that is, replacement is a word with a more specific, narrower meaning than fill-in:

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

Other hyponyms of fill-in include locum, locum tenens, double, alternate, surrogate.

fill-in (noun)	replacement (noun)
someone who takes the place of another (as when things get dangerous or difficult)	the act of furnishing an equivalent person or thing in the place of another
	a person or thing that takes or can take the place of another
	an event in which one thing is substituted for another
	a person who follows next in order
	someone who takes the place of another person
	filling again by supplying what has been used up