LDAP (Lightweight Directory Access Protocol)
The protocol and data model most on-premise directories speak — a hierarchical tree of entries, queried and authenticated over a standard wire protocol.
aka: LDAP
LDAP organizes identities and groups as a Directory Information Tree, where every entry has a distinguished name — its unique path read from the entry back to the root — and a set of attributes defined by a schema. It’s “lightweight” only relative to the heavier X.500 standards it descends from; three decades later it’s still the language on-premise directories, including Active Directory, speak underneath.