In cryptography, X.509 is an ITU-T standard for a public key infrastructure (PKI) and Privilege Management Infrastructure (PMI). X.509 specifies, amongst other things, standard formats for public key certificates, certificate revocation lists, attribute certificates, and a certification path validation algorithm.

Read more about X.509:  History and Usage, Certificates, Sample X.509 Certificates, Security, PKI Standards For X.509, Certification Authority, Public-Key Infrastructure (X.509) Working Group