Z39.50 is a national standard defining a protocol for computer-to-computer information retrieval. Z39.50 makes it possible for a user in one system to search and retrieve information from other computer systems (that have also implemented Z39.50) without knowing the search syntax that is used by those other systems. Z39.50 is an American National Standard that was originally approved by the National Information Standards Organization (NISO ) in 1988.
The legal definition of Z39.50 from the Library of Congress, Official Z39.50 maintenance Agency 
Northwestern University Library provides Z39.50 access to its OPAC and other Voyager databases. This connectivity can be utilized by other libraries and other Z39.50 client/server systems. Northwestern in conjunction with the Committee on Institutional Cooperation (CIC ) provides access to multiple academic library catalogs using the "Virtual Electronic Library ". You can also use many client based bibliographic tools to access our Voyager databases (e.g., Endnote, Bookwhere).