A nation is a peculiar sort of community that is easy to describe but hard to define. While nations can be described based on common cultural, historical and political institutions like shared religion, language, ethnicity or history, it is difficult to identify any defining features that a nation must possess.

For every possible criterion, there are exceptions and counter-examples. Many nations do not share a single common language or religion, while many shared languages or religions do not create a single unified nation. Conceptually, there is no hard distinction between a nation and other communities like ethnic groups or religious communities — any of these can one day form a nation, but no particular kind of community is guaranteed to form a nation.

The criterion that comes closest to distinguishing a nation is the state. Nations are communities that have a state of their own, which is why the two are joined with a hyphen to form the term nation-state. In modern times, there is generally a one-to-one bond between nation and state — one nation, one state; one state, one nation.

Today, a nation is best defined as a community that has succeeded in acquiring a state. Conversely, existing states also find it necessary to claim that they represent a nation. The nation has become the most accepted justification for a state, and states 'need' the nation as much as nations need states.