A library catalog (or library catalogue in British English) is a register of all bibliographic items found in a library or group of libraries, such as a network of libraries at several locations. A catalog for a group of libraries is also called a union catalog. A bibliographic item can be any information entity (e.g., books, computer files, graphics, realia, cartographic materials, etc.) that is considered library material (e.g., a single novel in an anthology), or a group of library materials (e.g., a trilogy), or linked from the catalog (e.g., a webpage) as far as it is relevant to the catalog and to the users (patrons) of the library.
The earliest library catalogs were lists, handwritten or enscribed on clay tablets and later scrolls of parchment or paper. As codices (books with pages) replaced scrolls, so too did library catalogs become like handwritten ledgers and, in some cases, printed books. During the late 18th century through mid-19th century, cataloguing on paper slips or cards gradually replaced ledgers and books as the main medium for library catalogs, and in the 20th it was long ubiquitous. The card catalog was a familiar sight to library users for generations. Computerized cataloguing developed gradually from the mid-20th, and by the late 20th and early 21st, it had mostly replaced card catalogs. The advent of the web brought about ubiquitous use of online public access catalogs (OPACs). Some people still informally refer to the online catalog as a "card catalog".
