In Windows NT and Windows 2000 networking, the domain controller (DC) is the server that responds to security authentication requests (logging in, checking permissions, etc.) within the Windows Server domain. It is the server that essentially makes networking (at least in a secure function) possible.

In Windows NT, the central domain controller was known as the Primary Domain Controller (PDC), of which there could only be one with this role; all other domain controllers were identified as Backup Domain Controllers (BDC). In Windows 2000, all domain controllers are theoretically equal, and therefore identified simply as Domain Controllers (DC). In practice however, this may not always be the case.

See Active Directory and Flexible single master operation for more on Windows 2000 domain controllers.