Information that has been determined pursuant to Executive Order 12958 as amended by Executive Order 13292, or any predecessor order, or by the Atomic Energy Act of 1954, as amended, to require protection against unauthorized disclosure and is marked to indicate its classified status.
Sources:
FIPS 200 under NATIONAL SECURITY INFORMATION
NIST SP 800-18 Rev. 1 under National Security Information
NIST SP 800-60 Vol. 1 Rev. 1 under National Security Information
NIST SP 800-60 Vol. 2 Rev. 1 under National Security Information

  Information that has been determined pursuant to Executive Order (E.O.) 13526 or any predecessor order to require protection against unauthorized disclosure and is marked to indicate its classified status when in documentary form.
Sources:
CNSSI 4009-2015 under classified national security information from E.O. 13526
NIST SP 800-37 Rev. 2 under classified national security information from CNSSI 4009-2015
NIST SP 800-53 Rev. 5 under classified national security information from E.O. 13526

  Information that Executive Order 13526, "Classified National Security Information," December 29, 2009 (3 CFR, 2010 Comp., p. 298), or any predecessor or successor order, or the Atomic Energy Act of 1954, as amended, requires agencies to mark with classified markings and protect against unauthorized disclosure. 
Sources:
CNSSI 4009-2015