LDAP (Lightweight Directory Access Protocol) is the standard protocol for centralized user and group management in enterprise environments. Stores users, groups, organizational structures in a hierarchical directory (tree structure). Active Directory (Microsoft) and OpenLDAP are common implementations. LDAP enables single sign-on (SSO)—one login for all applications. Applications query LDAP to authenticate users (verify credentials), authorize (check group membership), and retrieve user information (email, phone, department).