OPAC is an acronym for Online Public Access Catalog, which is an online catalog available to the public for searching and consulting bibliographic materials and more. In an OPAC, users can search and view information about books, journals, theses, maps, films and other materials available in a library or collection. Typically, OPACs provide detailed information about the title, author, publisher, year of publication, ISBN, description and other details of the material searched.

An OPAC can be used both by internal library users, such as students and researchers, and by external users who access the library online. The search interface can be customized according to the needs of the users. The OPAC can also provide information about loans, availability and expiration of materials, as well as the possibility of making online reservations.

OPACs are used primarily by libraries and academic institutions, but are also available to other organizations, such as museums, archives (including corporate archives) and research centers. OPACs are often integrated with other library management systems, such as book lending and returning, materials cataloging, and user management.