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.
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.