of a nation-state
A nation-state can be defined as a sovereign state with an essentially homogenous population and culture, like most (but not all) western European countries before post-colonial immigration waves. Under the state-sovereignty model of government and international relations, this was considered the optimal form for a state; people seem to like being governed by those like themselves. The physical extent of the population sharing a common culture (usually decided at its lowest level by language) serves as the base for constructing international borders and a system of government inside those borders.
A state-nation turns this idea on its head. In a state-nation, the state (as exemplified by the rules of governance and priorities set by those rules) is the basis for the common culture that it creates. Ethnicity, language and religion become much less of a factor in national identity, as they are subordinated to a belief in the underlying principles of the state as the designator of national identity.
The world's prime example of a state-nation is the United States of America; much of American culture is based on the ideals set forth in the Declaration of Independence ("that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness") and the Constitution (as amended). These ideals, not a shared language, religion or ethnicity, are what make the United States a nation rather than just a government covering a large land area.