A nation, in the sense of a common ethnicity, may include a diaspora or refugees who live outside the nation state; some nations of this sense do not have a state where that ethnicity predominates. In a more general sense, a nation state is simply a large, politically sovereign country or administrative territory. A nation state may be contrasted with:
A city-state, which is both smaller than a "nation" in the sense of "large sovereign country" and which may or may not be dominated by all or part of a single "nation" in the sense of a common ethnicity.
An empire, which is composed of many countries (possibly non-sovereign states) and nations under a single monarch or ruling state government.
A confederation, a league of sovereign states, which might or might not include nation-states.