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. Response:

A National Security System, as defined by various US federal laws and policies (e.g., FISMA), includes any information system operated by an agency or a contractor on behalf of an agency, the loss, misuse, or unauthorized access to or modification of which would compromise national security. This definition specifically covers the broad scope of systems mentioned in the question.

An Information Security Policy is a set of rules governing how an organization manages, protects, and distributes sensitive data, not a type of information system itself.

An Information System Owner is a role or individual responsible for a system, not the system itself.

System Security Authorization is the formal decision by an authorizing official to permit a system to operate, not the system itself.