X 500 directory
An X500 directory is called a Directory Information Base (DIB) or white pages. The program that maintains the DIBs is called a Directory Server Agent (DSA). A Directory Client Agent (DCA) is used to search DSA sites for names and addresses. It provides a hierarchical structure fitting the world's classification system: countries, states, cities, streets, houses, families, etc. The goal is to have a directory that can be used globally.

X 509 Certificate
A widely used specification for digital certificates.