Any information system (including any telecommunications system) used or operated by an agency or by a contractor of an agency, or other organization on behalf of an agency— (i) the function, operation, or use of which involves intelligence activities; involves cryptologic activities related to national security; involves command and control of military forces; involves equipment that is an integral part of a weapon or weapons system; or is critical to the direct fulfillment of military or intelligence missions (excluding a system that is to be used for routine administrative and business applications, for example, payroll, finance, logistics, and personnel management applications); or (ii) is protected at all times by procedures established for information that have been specifically authorized under criteria established by an Executive Order or an Act of Congress to be kept classified in the interest of national defense or foreign policy.
Sources:
FIPS 200 under NATIONAL SECURITY SYSTEM from 44 U.S.C., Sec. 3542
NIST SP 800-18 Rev. 1 under National Security System from 44 U.S.C., Sec. 3542

  (A) Any information system (including any telecommunications system) used or operated by an agency or by a contractor of an agency, or other organization on behalf of an agency— (i) the function, operation, or use of which— (I) involves intelligence activities; (II) involves cryptologic activities related to national security; (III) involves command and control of military forces; (IV) involves equipment that is an integral part of a weapon or weapons system; or (V) subject to subparagraph (B), is critical to the direct fulfillment of military or intelligence missions; or (ii) is protected at all times by procedures established for information that have been specifically authorized under criteria established by an Executive Order or an Act of Congress to be kept classified in the interest of national defense or foreign policy. (B) Subparagraph (A) (i) (V) does not include a system that is to be used for routine administrative and business applications (including payroll, finance, logistics, and personnel management applications).
Sources:
CNSSI 4009-2015 from 44 U.S.C., Sec. 3542 (b)(2)

  Any system (including any telecommunications system) used or operated by an agency or by a contractor of an agency, or other organization on behalf of an agency—(i) the function, operation, or use of which involves intelligence activities; involves cryptologic activities related to national security; involves command and control of military forces; involves equipment that is an integral part of a weapon or weapons system; or is critical to the direct fulfillment of military or intelligence missions (excluding a system that is to be used for routine administrative and business applications, for example, payroll, finance, logistics, and personnel management applications); or (ii) is protected at all times by procedures established for information that have been specifically authorized under criteria established by an Executive Order or an Act of Congress to be kept classified in the interest of national defense or foreign policy.
Sources:
NIST SP 800-37 Rev. 2 under national security system from 44 U.S.C., Sec. 3552
NIST SP 800-53 Rev. 5 under national security system from OMB Circular A-130 (2016)
NIST SP 800-53A Rev. 5 under national security system from OMB Circular A-130 (2016)
NIST SP 800-53B under national security system from OMB Circular A-130 (2016)

  Any information system (including any telecommunications system) used or operated by an agency or by a contractor of an agency, or other organization on behalf of an agency—(i) the function, operation, or use of which involves intelligence activities; involves cryptologic activities related to national security; involves command and control of military forces; involves equipment that is an integral part of a weapon or weapons system; or is critical to the direct fulfillment of military or intelligence missions (excluding a system that is to be used for routine administrative and business applications, for example, payroll, finance, logistics, and personnel management applications); or (ii) is protected at all times by procedures established for information that have been specifically authorized under criteria established by an Executive Order or an Act of Congress to be kept classified in the interest of national defense or foreign policy.
Sources:
NIST SP 800-137 under National Security System from 44 U.S.C., Sec. 3542

  Any information system (including any telecommunications system) used or operated by an agency or by a contractor of an agency, or other organization on behalf of an agency (i) the function, operation, or use of which involves intelligence activities; involves cryptologic activities related to national security; involves command and control of military forces; involves equipment that is an integral part of a weapon or weapons system; or is critical to the direct fulfillment of military or intelligence missions (excluding a system that is to be used for routine administrative and business applications, for example, payroll, finance, logistics, and personnel management applications); or (ii) is protected at all times by procedures established for information that have been specifically authorized under criteria established by an Executive Order or an Act of Congress to be kept classified in the interest of national defense or foreign policy.
Sources:
NIST SP 800-30 Rev. 1 under National Security System from 44 U.S.C., Sec. 3542
NIST SP 800-39 under National Security System from 44 U.S.C., Sec. 3542

  Any information system (including any telecommunications system) used or operated by an agency or by a contractor on behalf of an agency, or any other organization on behalf of an agency – (i) the function, operation, or use of which: involves intelligence activities; involves cryptologic activities related to national security; involves command and control of military forces; involves equipment that is an integral part of a weapon or weapon system; or is critical to the direct fulfillment of military or intelligence missions (excluding a system that is to be used for routine administrative and business applications, for example payroll, finance, logistics, and personnel management applications); or (ii) is protected at all times by procedures established by an Executive order or an Act of Congress to be kept classified in the interest of national defense or foreign policy.
Sources:
NIST SP 800-60 Vol. 1 Rev. 1 under National Security System from 44 U.S.C., Sec. 3542
NIST SP 800-60 Vol. 2 Rev. 1 under National Security System from 44 U.S.C., Sec. 3542

  (A) Any information system (including any telecommunications system) used or operated by an agency or by a contractor of an agency, or other organization on behalf of an agency— (i) the function, operation, or use of which— (I) involves intelligence activities; (II) involves cryptologic activities related to national security; (III) involves command and control of military forces; (IV) involves equipment that is an integral part of a weapon or weapons system; or (V) subject to subparagraph (B), is critical to the direct fulfillment of military or intelligence missions; or (ii) is protected at all times by procedures established for information that have been specifically authorized under criteria established by an Executive Order or an Act of Congress to be kept classified in the interest of national defense or foreign policy. (B) Subparagraph (A) (i) (V) does not include a system that is to be used for routine administrative and business applications (including payroll, finance, logistics, and personnel management applications).
Sources:
NIST SP 800-59 under national security system from 44 U.S.C., Sec. 3542 (b)(2)