What is Nation State?

Legal Definition
A nation state is a type of state that joins the political entity of a state to the cultural entity of a nation, from which it aims to derive its political legitimacy to rule and potentially its status as a sovereign state if one accepts the declarative theory of statehood as opposed to the constitutive theory. A state is specifically a political and geopolitical entity, whilst a nation is a cultural and ethnic one. The term "nation state" implies that the two coincide, in that a state has chosen to adopt and endorse a specific cultural group as associated with it. "Nation state" formation can take place at different times in different parts of the world.

The concept of a nation state can be compared and contrasted with that of the multinational state, city state, empire, confederation, and other state formations with which it may overlap. The key distinction is the identification of a people with a polity in the "nation state."
-- Wikipedia