Wiktionary
online public access catalog
n. (context library science English) A computerized online publicly accessible catalog of the materials held in a library, or library system.
Wikipedia
Online public access catalog
An online public access catalog (often abbreviated as OPAC or simply library catalog) is an online database of materials held by a library or group of libraries. Users search a library catalog principally to locate books and other material available at a library. In simple language it is an electronic version of the card catalogue. OPAC is the gateway to library's collection.