A card index is a number of cards with information written on them which are arranged in a particular order, usually alphabetical, so that you can find the information you want easily.
card index in British English
or card file
noun
1.
an index in which each item is separately listed on systematically arranged cards
verbcard-index
2. (transitive)
to make such an index of (a book)
card-index in British English
(ˈkɑːdˌɪndɛks )
verb(transitive)
to make a card index of (a book)
It is easier to card-index a man's bank balances than his fundamental values.
card index in American English
US
card file
Examples of 'card index' in a sentence
card index
My private life was like a card index; he flicked through it with an ease born of long practice.
Taylor, Andy TOY SHOP
In the coach she indicated a card index at the side of the counter.
Forsythe, Malcolm THE BOOK LADY
Then he turned to the card index and tore out the entry for Matthew Holmwood.
Stewart, Michael GRACE
Her card-index, consulted no doubt on the night before the fête, must have been put out of her mind immediately afterwards.