An OPAC (Online Public Access Catalog) is an online bibliography of a library collection that is available to the public. OPACs developed as stand-alone online catalogs, often from VT100 terminals to a mainframe library catalog. With the arrival of the Internet, most libraries have made their OPAC accessible from a server to users all over the world.Content Continues Below
User searches of an OPAC make use of the Z39.50 protocol. This protocol can also be used to link disparate OPCS into a single "union" OPAC.