X.500 is a series of computer networking standards covering electronic directory services. The X.500 series was developed by ITU-T, formerly known as CCITT, and first approved in 1988. The directory services were developed in order to support the requirements of X.400 electronic mail exchange and name lookup. ISO was a partner in developing the standards, incorporating them into the Open Systems Interconnection suite of protocols. ISO/IEC 9594 is the corresponding ISO identification.
Read more about X.500: X.500 Protocols, X.500 Data Models, The Relationship of The X.500 Directory and X.509v3 Digital Certificates, List of X.500 Series Standards, Criticism