V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

BDC
       Backup Domain Controller (MS, Windows NT, PDC)


The Free On-line Dictionary of Computing (30 December 2018):

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)