Backup Domain Controller

Backup Domain Controller - One or more computers running Windows NT that act as a backup to the primary domain controller (PDC). These machines can authenticate security requests just like the primary domain controller, and will take over for the PDC if it is not available. However, a BDC will remain a BDC until it is manually promoted to a PDC by a domain administrator. With Active Directory Windows no longer requires the sometimes confusing primary and backup domain controllers, just domain controllers instead.