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

directory service

    A structured repository of information
   on people and resources within an organisation, facilitating
   management and communication.

   On a LAN or WAN the directory service identifies all
   aspects of the network including users, software, hardware,
   and the various rights and policies assigned to each.  As a
   result applications can access information without knowing
   where a particular resource is physically located, and users
   interact oblivious to the network topology and protocols.

   To allow heterogeneous networks to share directory
   information the ITU proposed a common structure called
   X.500.  However, its complexity and lack of seamless
   Internet support led to the development of Lightweight
   Directory Access Protocol (LDAP) which has continued to
   evolve under the aegis of the IETF.  Despite its name LDAP
   is too closely linked to X.500 to be "lightweight".

   LDAP was adopted by several companies such as Netscape
   Communications Corporation (Netscape Directory Server) and
   has become a de facto standard for directory services.
   Other LDAP compatible offerings include Novell, Inc.'s
   Novell Directory Services (NDS) and Microsoft
   Corporation's Active Directory.  The Netscape and Novell
   products are available for Windows NT and Unix
   platforms.  Novell Directory Services also run on Novell
   platforms. Microsoft Corporation's Active Directory is an
   integral part of Microsoft's Windows 2000 and although it
   can interface with directory services running on other systems
   it is not available for other platforms.

   (2001-01-02)