Nation seems a pretty ambiguous concept. Some large group associated by some set of shared attributes: territory, language, religion….but those are not exclusive or required.
A sovereign nation state is, however, well defined when it is recognised as such by other nation states. Sovereignty. They are usually defined by territory marked by borders, constitutions, governments and legal frameworks that recognise citizenship.
Nation states are a fairly recent political invention, being not more than a few hundred years old, a European construction? That is open for debate. Peering into history, I am sure we can find many examples of states in ancient times. The Greek city states is an obvious example. Larger nation states became possible once transport technology had evolved sufficiently for people to move around easily. If people never move far from your place of their birth, they develop very localised customs, culture and languages.
Nation states were often built on older political structures like Kingdoms and Empires that had some centralised political authority, but would often be quite unstable. Many modern states were created by consolidating large numbers of smaller kingdoms.
Nation states sometimes break apart because groups within the state decide they want independence and their own sovereignty. Most nation states are troubled by constitutional fault lines that sometime cause them to break apart either peacefully or painfully in bad civil wars.
Nation states often try hard to develop a positive, heroic ‘national identity’ with lots of dancing around flags romantic foundation myths in order to encourage people to accept the central authority of the government. They create patriotic heroes made famous by their triumphs over rival states. Sometimes they try to deal with threats by classifying people according to some criteria. Who is a state national and who is not and allocate rights and privileges to those that are ‘in’ and marginalise those that are ‘out’. Some people can end up as ‘stateless’ and this is rarely a good situation. There are some large groups that a stateless and exist as minorities within larger states. This precarious status is a big disadvantage and has led to persecution and tragedy.
This is politics, but defining a group is much more than that. It can be emotional, cultural, religious. Assigning ‘nation’ to an identity suggests it is on the scale of viable state. It could refer to the old kingdoms on which a state was built. That puts a national identity in competition with a political identity. Or perhaps it is complimentary to it.
National identity can be a whole package of attributes and styles. If we take the political requirements of the nation state to separate sheep from goats out if it what are we left with?
A cultural affinity, a language group, a religion? All of these things do not make it easy to define a nation. The place of your birth? Some people really wanted to escape from that place as soon as they were able and would resent the label. National border have tendency to change pretty frequently.
Do you get to choose to be a member of a nation, like a club membership or is it thrust upon you whether you like it or not?
Nation, outside the current political definition of a nation state and citizenship thereof, is far too ambiguous a term to be useful. Identity is so much more complex.