An application layer protocol (see seven-layer reference model) to query and control X500 directory services on a TCP/IP network. It was developed in the early 1990s as an alternative to the X500 Directory Access Protocol, being ‘lightweight’ in the sense that it that did not require an open systems interconnection protocol stack. The current version is LDAPv3, defined in RFC 4510.

http://tools.ietf.org/html/rfc4511 The LDAP specification