The Free On-line Dictionary of Computing (19 January 2023):

Backup Domain Controller
BDC

    (BDC) A server in a network of Microsoft
   Windows computers that maintains a copy of the SAM database
   and handles access requests that the Primary Domain
   Controller (PDC) doesn't respond to.  There may be zero or
   more BDCs in a network.  They increase reliability and reduce
   load on the PDC.

   (2006-09-18)