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. Note: Synonymous with classified national security information (CNSI) (see E.O. 13526). Often used interchangeably with national security information (NSI) (see E.O. 13526).
Sources:
CNSSI 4009-2022 under national security information (NSI) from E.O. 13526 - adapted

  Information that has been determined pursuant to Executive Order 13526 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 800-39 under Classified National Security Information from CNSSI 4009

  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:
NIST SP 800-37 Rev. 2
NIST SP 800-53 Rev. 5 from E.O. 13526