Any activity or effort performed to protect a nation against attack or other threats.

Ostensibly, the term should be equivalent to literal military defense, but it may also be applied to offensive actions or policies, or to non-military actions or policies.

For the United States, "National defense" is the name of a area of 'tax outlay' (a division of how tax income is spent; other outlays are "Medicare", "Social Security", and "Non-defense discretionary spending").