The nation-state is a certain form of state that derives its political legitimacy from serving as a sovereign entity for a nation as a sovereign territorial unit. The state is a political and geopolitical entity; the nation is a cultural and/or ethnic entity. The term "nation-state" implies that the two geographically coincide, and this distinguishes the nation state from the other types of state, which historically preceded it.

Due to ambiguities in the word state, for instance in the United States, the term nation-state is also used to mean any sovereign state, whether or not its political boundaries coincide with ethnic and cultural ones. The usage appears to arise from an attempt to distinguish a sovereign nation-state from a federal state — that is a subordinate member of a federal system — such as a U.S. State. Ambiguities in the usage of terms such as nation, international, state, and country, are discussed at Nation.