The Free On-line Dictionary of Computing (30 December 2018):

ANSI Z39.50
Z39.50

    Information Retrieval Service
   Definition and Protocol Specification for Library
   Applications, officially known as ANSI/NISO Z39.50-1992, and
   ANSI/NISO Z39.50-1995.  This standard, used by WAIS,
   specifies an OSI application layer service to allow an
   application on one computer to query a database on another.

   Z39.50 is used in libraries and for searching some databases
   on the Internet.  The US Library of Congress
   (http://lcweb.loc.gov/z3950/agency/) is the official
   maintanence agency for Z39.50.

   Index Data, a Danish company, have released a lot of Z39.50
   code.  Their website explains the relevant ISO standards
   and how they are amicably converging in Z39.50 version 4.0.

   Overview (http://nlc-bnc.ca/ifla/VI/5/op/udtop3.htm).

   Z39.50 resources
   (http://lamp.cs.utas.edu.au/net.html#Z3950).

   (1996-07-22)