replacement vs fill-in

replacement and fill-in both are nouns.

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

As nouns, fill-in is a hypernym of replacement; that is, fill-in is a word with a broader meaning than replacement:

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

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

replacement (noun)	fill-in (noun)
the act of furnishing an equivalent person or thing in the place of another	someone who takes the place of another (as when things get dangerous or difficult)
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