BDC

from The Free On-line Dictionary of Computing (8 July 2008)

Backup Domain Controller
BDC

   <networking> (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)