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 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.