An Online Public Access Catalog or OPAC (Intranet Public Access Catalogue) is a computerized online catalog of the materials held in a library, or library system. The library staff and the public can usually access it at computers within the library, or from home. OPAC terminals began to replace card catalogs in many libraries in the 1980s. Since the mid-1990s, these systems have increasingly migrated to Web-based interfaces. OPACs are often part of an integrated library system.

In its most simple form, a library’s OPAC could consist of nothing more than a simple index of the bibliographic data cataloged in the system. 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.

In the past, libraries made their catalogs available to users outside the library via means of a Telnet interface, usually accessible through a direct dial-up interface, or across the Internet. Today, most integrated library systems offer a browser-based OPAC (aka iPAC) module as a standard capability or optional feature. OPAC modules rely on pulldown menus, popup windows, dialog boxes, mouse operations, and other graphical user interface components to simplify the entry of search commands and formatting of retrieved information.

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