LDAP stands for Lightweight Directory Access Protocol. The lightweight part comes from the fact it does not implement ALL of the standard. X.500 is quite unwieldy, and much of it has been superseded (e.g. the OSI networking stack) or considered impractical (e.g. such as canonical per entry Distinguished Names) or undesirable (e.g. publishing directories of private entities like businesses).
Found via a link on ... to https://wiki.gentoo.org/wiki/Project:Prefix