This is one of the methods of indexing. It is a file or catalog consisting of cards on which information has been entered so that desired items or data can be readily found. Under the system, a card containing the name, address and the holder number of the correspondent is kept in separate card drawers either loose or inserted on a rod passing through the drawer. It is a collection of such cards either serves as or aids the creation of, an index for expedited lookup of information (such as a library catalog or a back-of-the-book index).
Definition: 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. The cards are arranged alphabetically, numerically or geographically. So, it is a set of cards that have information written on them and are arranged in alphabetical order.
Index cards are used for a wide range of applications and environments: in the home to record and store recipes, shopping lists, contact information, and other organizational data; in business to record presentation notes, project research and notes, and contact information. In schools as academic research to hold data such as bibliographical citations or notes, here it is a box of cards that contain information about something and are arranged in order, especially the cards with book information on them in a library.