A nation-state is a political unit where the state and nation are congruent. It is a more precise concept than "country", since a country does not need to have a predominant ethnic group.

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 multinational state, where no one ethnic group dominates (such a state may also be considered a multicultural state depending on the degree of cultural assimilation of various groups).
• A city-state, which is both smaller than a "nation" in the sense of a "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.
• A federated state, which may or may not be a nation-state, and which is only partially self-governing within a larger federation (for example, the state boundaries of Bosnia and Herzegovina are drawn along ethnic lines, but those of the United States are not).

This article mainly discusses the more specific definition of a nation-state as a typically sovereign country dominated by a particular ethnicity.