Short for Backup Domain Controller. BDCs maintain a read-only backup of the PDC's master directory. Although BDCs cannot be changed, they can be upgraded to PDCs and regularly synchronized with PDCs.
Windows 2000 (W2K) approaches the domain controller concept a little differently. Unlike Windows NT, where a PDC must be accessible to make changes to the directory, W2K relies on Active Directory's multimaster replication model to update all domain controllers when only one is updated. There is no PDC or BDC. All domain controllers are equal.