Defense-in-Depth

The application of multiple countermeasures in a layered or stepwise manner to achieve security objectives. The methodology involves layering heterogeneous security technologies in the common attack vectors to ensure that attacks missed by one technology are caught by another.

See NISTIR 8183 under Defense-in-depth (ISA/IEC 62443)  for more information.

Information security strategy integrating people, technology, and operations capabilities to establish variable barriers across multiple layers and missions of the organization. 

For more information, see:

NIST SP 800-53 Rev. 4 under Defense-in-Depth
NIST SP 800-30 Rev. 1 under Defense-in-Depth (CNSSI 4009)
NIST SP 800-39 under Defense-in-Depth (CNSSI 4009)
NIST SP 800-53 Rev. 4 under Defense-in-Depth
CNSSI 4009-2015 (NIST SP 800-53 Rev. 4)
NIST SP 800-161 under Defense-in-Depth (CNSSI 4009, NIST SP 800-53 Rev. 4)
NISTIR 7622 under Defense-in-Depth (CNSSI 4009-2010, NIST SP 800-53)