A catalog listing all the holdings of a library. A library catalog is an index into these holdings, because when you locate something in the catalog, it will tell you how to (physically or electronically) find that item. So, it's more than a simple list.

In the past, library catalogs were always card catalogs, with all the books, magazines and so on typed on index cards -- maybe even 10 or 20 per book! -- and organized in little drawers. There would be one card for the Title index, one for each author for the Author index, and often multiple cards for the Subject index (depending on how many subjects the item is classified under). Gawd, what a lot of work!

These days, library catalogs are maintained and accessed electronically. It's still alot of work, because the number of information-bearing things in the world keeps increasing exponentially.