Z39.50
Z39.50 is a standard communications protocol for the search and retrieval of bibliographic data in online databases. Z39.50 is used on the Internet to search the Online Public Access Catalogues (OPAC) of library holdings. It is also sometimes used to link disparate OPACs into a single "union" OPAC. Z39.50 is an American National Standards Institute (ANSI/NISO) standard.