Answer:
A nation-state is a state where the vast majority of the population is comprised of a single ethnic group which also serves as a motivation and reason for the formation of the state.
Explanation:
A nation-state is a term used for the nations where the vast majority of the population is comprised of a single ethnic group, usually over 90%. There are multiple examples of such states around the world, some of which are fully recognized, while some function in practice as such but are not officially recognized by the United Nations. In the first category, we have the likes of Mongolia, Iceland, and numerous island nations in the Pacific, while in the second one we have the likes of Kosovo and South Ossetia.
These nations are often formed on the basis of the distribution of an ethnic group, thus determining the boundary in accordance with where the particular ethnic group is found without interruptions. An interesting case was the recent referendum for independence in Scotland as if it was successful it would have meant the creation of another nation-state. This was carefully monitored by many ethnic groups in the world because they saw it as an opportunity to create their own nation-states.