"Country", "State" and "Nation" are often used synonymously to refer to political entities. But if you want to nitpick:
- A "country" usually means a geographical region.
- A "state" is a political organization which rules over a country.
- A "nation" is a far more fuzzy term. It usually means a group of people who are connected by culture and heritage. Some people might also consider common ethnicity to be a descriptor of nationality, but how ethnically homogeneous a group of people needs to be to be considered one nation is a matter of debate.
So People can reside in a country, but you can not be part of a country. Mountains, forests or cities can be part of a country, but people can not because people aren't geographical places.
For example, let's say you are a citizen of Arstotzka, but you currently work and live in Kolechia as a guest worker.
- You are currently present in the country of Kolechia.
- As long as you are in the country of Kolechia, you have to follow Kolechian laws, so you are a subject of the state of Kolechia. You would also be counted as part of the population of Kolechia.
- But as long as you speak Arstotzkan, live a typical Arstotzkan lifestyle, self-identify as an Arstotzkan and still have Arstotzkan citizenship, you are still part of the Arstotzkan nation.
A nation state is a state where the majority of the population is considered to be part of one nation and where no notable numbers of that nation live outside of the state. Real world counter-examples are:
- Cases where nations are split by state borders. For example, the Korean nation is currently split across North Korea and South Korea.
- Cases where one state governs people who consider themselves different nations. For example, a lot of citizens of the United Kingdom of Great Britain and Northern Ireland usually don't self-identfy as "United Kingdomers" but rather as "English", "Welsh", "Scottish" or "North-Irish" (although some people in Northern Ireland might consider themselves part of the "Irish" nation which is split between the Republic of Ireland and the United Kingdom).
However, I would not be surprised if some people would dispute these two examples. You could claim that the people who live in North Korea and South Korea have diverged so far in their culture that they can no longer be considered one nation. One could also claim that the Scottish, Welsh, English and North-Irish are subjects of the United Kingdom for so long that they are now just different ethnic groups within one "British" nation.
Which groups of people are the same nation and which are not can be a very subjective distinction. And people often try to establish their preferred definitions to achieve political goals. For example, when a region of a country is having a referendum for independence, then the proponents might claim "We should be independent because we are a separate nation" while the opponents might claim "We should stay together because we are part of one big nation". Who of them is right? That's a matter of personal opinion.