X509 is the name of the standard which defines Public Key Infrastructure and the format of SSL/TLS Certificates. My question is: What does x509 mean?
X.509 is a standard number/index defined by ITU-T, telecommunication and standartization commitee.
X stands for Data networks, open system communications and security standards track and every standard has a number. X.500-X.599 standards define directory services (such as DAP, LDAP, etc.). And 509 is just a number of standard in X track.
According to Wikipedia:
X.509 was initially issued on July 3, 1988 and was begun in association with the X.500 standard.
X.500 is a series of computer networking standards covering electronic directory services. The X.500 series was developed by the Telecommunication Standardization Sector of the International Telecommunications Union (ITU-T). ITU-T was formerly known as the Consultative Committee for International Telephony and Telegraphy (CCITT).
X.500 was approved first in 1988. The directory services were developed to support requirements of X.400 electronic mail exchange and name lookup. The International Organization for Standardization (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.
X.400 is a suite of ITU-T Recommendations that defines the ITU-T Message Handling System (MHS).
I could not find why X.400 is named X.400 though, also why X509 and not X.502.