The CCITT's definition of X500 is:
"a collection of open systems which co-operate to hold a logical data base of information about the set of objects in the real world. The users of the Directory, including people and computer programs, can read or modify the information, or parts of it, subject to having permission to do so."
The CCITT called the Directory in the singular to reflect the intention to create a single logical directory.
Basically X500 defines:
X500 does not define the user interfaces nor does it define the implementation technology.
In practical terms, what the X500 offers is the access to a distributed open, online directory. Thus, while the initial drive for X500 is to provide directory support for the X400 MHS services, X500 also offers an open standard for a generalised directory capability and it's possible use extends beyond X400 and OSI applications. Today, X500 is seen potentially as providing support for:
While the X500 directory is similar in many ways to a general purpose database management system, it is not designed to be one, although the X500 directory may itself be built on top of such a general-purpose database system.