substitution vs fluctuation

substitution and fluctuation both are nouns.

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

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

Other hypernyms of substitution include variation.

substitution (noun)	fluctuation (noun)
the act of putting one thing or person in the place of another:	the quality of being unsteady and subject to changes
an event in which one thing is substituted for another	an instance of change; the rate or magnitude of change
	a wave motion