What is Backup Domain Controller (BDC)?

In Windows NT Server 4.0 or earlier, a computer running Windows NT Server that receives a copy of the domain's directory database (which contains all account and security policy information for the domain).
The copy is synchronized periodically and automatically with the master copy on the primary domain controller (PDC). BDCs also authenticate user logon information and can be promoted to function as PDCs as needed. Multiple BDCs can exist in a domain. Windows NT 3.51 and 4.0 BDCs can participate in a Windows 2000 domain when the domain is configured in mixed mode.