replacement vs substitute

replacement and substitute both are nouns.

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

As nouns, replacement and substitute are synonyms defined as:

replacement and substitute: a person or thing that takes or can take the place of another

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

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

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

replacement (noun)	substitute (noun)
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
a person or thing that takes or can take the place of another	someone who takes the place of another (as when things get dangerous or difficult)
an event in which one thing is substituted for another	an athlete who plays only when a starter on the team is replaced
a person who follows next in order
someone who takes the place of another person
filling again by supplying what has been used up

replacement (adjective)	substitute (adjective)
	artificial and inferior
	serving or used in place of another
	capable of substituting in any of several positions on a team

replacement (verb)	substitute (verb)
	put in the place of another; switch seemingly equivalent items
	be a substitute
	act as a substitute