Z39.50 (z3950) is a protocol. It is also a NISO standard, developed by the library community and used to create a client application that allows users to search and retrieve records within a bibliographic database. Z39.50 was originally developed to allow libraries to search each other's OPACs, even when they were provided by different vendors. In the past few years, the use of Z39.50 has been expanded to access A&I database (like those on EBSCOhost) as well as full text resources (also like those on EBSCOhost).
The Z39.50 client application may take the form of a standalone program, or it may be a web gateway. With Z39.50-enabled web gateways, the library can create their own web interfaces to library catalogs and databases such as EBSCOhost.
The US Library of Congress provides the following document on the Z39.50 protocol and application service definition: