Online Public Access Catalog (OPAC): An Online Public Access Catalog (OPAC) is a computerized online catalogue of the materials held in a library, or a library system. OPACs are often part of an integrated library management system or software. The library staff and the public can usually access it in computers within the library, or from home via the Internet. Since the mid-1980s, it has replaced the card catalog in most libraries. Since the mid-1990s, character-based OPAC interfaces are being replaced by Web-based interfaces. Today more complex OPACs offer a variety of search capabilities on several indexes, integrate rich content (book covers, video clips, etc.), and offer interactive request and renewal functionality.
The OPAC has many advantages over card catalogues. It can store entries; it can add new entries, withdraw entries and print out updated version of a catalogue in book, card or shelf form. It can also be used to search and produce catalogue in CD, DVD, etc. It itself can be used as a catalogue cabinet with enhance features i.e information can be stored within the computer and kind of entries required can be easily got as and when required. It has also the facility to input the data from the point of origin and output data can be transmitted directly to the place where it is needed by using teleprocessing.
OPAC is more useful than the traditional card formats because:
i) The online catalogue does not need to be sorted statically. Here the user can choose the author, title, keyword, or systematic order dynamically.
ii) Most online catalogue offer search facility for any word of the title. The goal of the grammatical word order which is to provide an entry on the word that most users would look for is reached even better.
iii) Many online catalogues allow links between several variants of an author’s name. So, authors can be found both under the original and the standardized name (if entered properly by the cataloguer).
Online cataloguing has greatly enhanced the usability of catalogues, and its origin from the effort of MAchine Readable Cataloging (MARC) standards in the 1960s. The rules governing the creation of catalogue MARC records include not only formal cataloging rules like AACR2 but also the special rules specific to MARC, available from the Library of Congress and also OCLC.