Domain join is the state where a Windows device is enrolled in an enterprise directory domain, such as Active Directory, and uses that domain for authentication and policy enforcement. Instead of relying only on a local account, the device trusts domain services to validate sign-in and deliver rules such as password policy, access controls, and managed settings.
This matters because many attacks and defenses depend on that trust relationship. Attackers may try to abuse domain-joined endpoints to steal credentials, pivot through the network, or weaken the secure channel that supports logon. Defenders rely on domain join to centralize identity and hardening, but updates, misconfigurations, or incompatible policies can break sign-in even when the password is correct. When domain join fails, users may be locked out of the desktop, VPN, and other enterprise apps that depend on it.