back matter vs index

back matter and index both are nouns.

Word	Noun	Adjective	Verb	Adverb
back matter	Yes	No	No	No
index	Yes	No	Yes	No

back matter (noun)	index (noun)
written matter following the main text of a book	the finger next to the thumb
	an alphabetical listing of names and topics along with page numbers where they are discussed
	a number or ratio (a value on a scale of measurement) derived from a series of observed facts; can reveal relative changes as a function of time
	a mathematical notation indicating the number of times a quantity is multiplied by itself
	a numerical scale used to compare variables with one another or with some reference number