card catalog vs card index

card catalog and card index both are nouns.

Word	Noun	Adjective	Verb	Adverb
card catalog	Yes	No	No	No
card index	Yes	No	No	No

As nouns, card catalog and card index are synonyms defined as:

card catalog and card index: an alphabetical listing of items (e.g., books in a library) with a separate card for each item

Other synonyms of card catalog include card catalogue.

card catalog (noun)	card index (noun)
an alphabetical listing of items (e.g., books in a library) with a separate card for each item	an alphabetical listing of items (e.g., books in a library) with a separate card for each item
a library catalog in which each publication is described on a separate file card