 A "domain" is a concept introduced in Windows NT whereby a user may be granted access to a number of computer resources with the use of a single username and password combination.With Windows NT 4 Server, one domain controller per domain was configured as the primary domain controller (PDC); all other domain controllers were backup domain controllers (BDC).Because of the critical nature of the PDC, best practices dictated that the PDC should be dedicated solely to domain services, and not used for file, print or application services that could slow down or crash the system.