National Security System (NSS)
Any information system (including any telecommunications system) used or operated by an agency or a contractor of an agency, or other organization on behalf of an agency, the function, operation, or use of which:
(1) involves intelligence activities;
(2) involves cryptologic activities related to national security;
(3) involves the command and control of military forces;
(4) involves equipment that is an integral part of a weapons or weapons system; or
(5) is critical to the direct fulfillment of military or intelligence missions.
Item (5) above does not include a system that is to be used for routine administrative and business applications (including payroll, finance, logistics, and personnel management applications).
Source: DoDI 8000.01