A nation-state is a state whose borders and (originally) citizenship are largely defined by a singular nationality. Israel and Japan, for example. Belgium and Canada are not nation states: they are split into French and Flemish, and Anglo and French nationalities, respectively.

It is a 19th century term that rarely applies these days, but still sees some residual usage.

To complete the other half of the story for those not familiar: most all infosec references to "nation-state attack" instead use it to mean "government backed attack" (regardless if a nation-state is involved in the context).

Anglo, French _and Aboriginal_ nationalities. That's not a minor detail to exclude.