Classified information or classified national security information means information that has been determined pursuant to E. O. 12958 as amended by E.O. 13292 or any predecessor order to require protection against unauthorized disclosure and is marked to indicate its classified status when in documentary form.
Sources:
NIST SP 800-59 under Classified Information from E.O. 13292

  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
NIST SP 800-37 Rev. 2
NIST SP 800-53 Rev. 5

  Information that has been determined pursuant to Executive Order (E.O.) 13292 or any predecessor order to require protection against unauthorized disclosure and is marked to indicate its classified status when in documentary form.
Sources:
NIST SP 800-60 Vol. 1 Rev. 1 under Classified Information
NIST SP 800-60 Vol. 2 Rev. 1 under Classified Information