<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)

<networking> (PDC) Each Windows NT domain has a Primary Domain Controller and zero or more {Backup Domain Controllers}. The PDC holds the SAM database and authenticates access requests from workstations and servers in the domain. (2003-07-16)

<operating system> ("back up" when used as a verb) A spare copy of a file, file system, or other resource for use in the event of failure or loss of the original. The term commonly refers to a copy of the files on a computer's disks, made periodically and kept on {magnetic tape} or other removable medium (also called a "dump"). This essential precaution is neglected by most new computer users until the first time they experience a disk crash or accidentally delete the only copy of the file they have been working on for the last six months. Ideally the backup copies should be kept at a different site or in a fire safe since, though your hardware may be insured against fire, the data on it is almost certainly neither insured nor easily replaced. See also backup software, differential backup, incremental backup, full backup. Compare archive, source code management. (2004-03-16)