replacement vs filling

replacement and filling both are nouns.

Word	Noun	Adjective	Verb	Adverb
replacement	Yes	No	No	No
filling	Yes	No	No	No

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

replacement (noun)	filling (noun)
the act of furnishing an equivalent person or thing in the place of another	the act of filling something
a person or thing that takes or can take the place of another	the yarn woven across the warp yarn in weaving
an event in which one thing is substituted for another	(dentistry) a dental appliance consisting of any of various substances (as metal or plastic) inserted into a prepared cavity in a tooth
a person who follows next in order	a food mixture used to fill pastry or sandwiches etc.
someone who takes the place of another person	flow into something (as a container)
filling again by supplying what has been used up	any material that fills a space or container