Active Directory (AD)
Microsoft's on-premise directory product — LDAP plus Kerberos authentication, Group Policy, and a domain/forest structure for managing Windows-joined machines.
aka: AD
AD is one of the most deployed systems that speaks LDAP, but it’s more than the protocol: Kerberos handles authentication, Group Policy enforces machine configuration, and domains and forests give large organizations a way to delegate administration without flattening everything into one directory. A compromised domain controller means an attacker can impersonate anyone in it — which is why domain-controller security sits among the highest-value defenses in IAM.