National security is the requirement to maintain the survival of the state through the use of economic, military and political power and the exercise of diplomacy.
Wikipedia goes on to state that no generally accepted definition exists and lists a couple of alternative definitions.
I paid attention to this because I was repeatedly flabbergasted by responses to "national security policy" or "defence policy" topics and discussions that asserted the need for great power games in this or that region.
In my opinion, that's (stupid) foreign policy, but neither national security nor defence policy.
I consider defence policy to be the (military) subset of national security policy, and would define the latter as
National security policy is a policy to provide security without compromising sovereignty primarily and to maintain a good foreign political freedom of action secondarily.
To provide security means to minimise (net) harm done to the nation, and since war equals usually (major risk of) harm done to your nation, a successful national security policy is basically about maintaining peace.
Warfare is either an indicator of failure or a result of extraordinarily bad circumstances (such as an aggressor hell-bent on war).
Great power games are not about security; they are at best about saving raw material import costs. A recurring problem of great power games is that the costs are typically higher than the benefit even in the event of success.