National security, in the most objective usage, describing the totality of necessary functions of a nation; maintaining it is the mirror of the grand strategic attempts of adversaries to disrupt a nation. It also can reasonably refer to domestic attempts, as in insurgency, to disrupt a government, economy, or culture.
It can have a reasonable use in broad legal policy, as in defining the potential and appropriate use of a state secrets privilege. It can also be used as a political or demagogic appeal to allow unrestrained state authority. In some cases, it is used in an apparently euphemistic way that remains informative to persons aware of the context, such as the nondescript name of the U.S. National Security Agency.