replacement vs substitution

replacement and substitution both are nouns.

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

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

replacement: the act of furnishing an equivalent person or thing in the place of another
substitution: the act of putting one thing or person in the place of another:

Other hypernyms of replacement include commutation, exchange.

As nouns, replacement and substitution are synonyms defined as:

replacement and substitution: an event in which one thing is substituted for another

Other synonyms of replacement include permutation, switch, transposition.

replacement (noun)	substitution (noun)
the act of furnishing an equivalent person or thing in the place of another	the act of putting one thing or person 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
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