A nation is a group of people identifying with one another, recognising a shared connection derived from ancestry, kinship, location, language, culture or belief. Or perhaps they are just ruled by the same government. Or decisive historic events welded them into a nation, willingly or not.
We could define three main kinds of nation: 'first nations', 'nations' and 'states'.
Nationhood is complex. There are some 180 states in the world, and some 15,000 identifiable peoples. Some peoples are spread across different states, and some states encompass different peoples. This becomes a problem when one people feels oppressed. Which is common.
First nations are conglomerations of clans and tribes sharing a native identity, culture and origin. They usually trace their heritage back to a common ancestor or source. Most first nations existed before recorded history, possessing an inherent natural, ethnic nationhood.
Nations are variously welded together by historical events and ruling élites. They possess a created emotional integrity. Sometimes nations possess sufficient social glue to hang together, and sometimes not or they tread a narrow line between institutional unity and human diversity. Before states arose, nations had heartlands without definitive boundaries, held together by traditional power-arrangements, custom or kinship.
States are governmental entities formed by rulers, élites or political agreements, largely for consolidating power, often without the consent of subject peoples. The first ancient states were city-states or empires. States establish clear boundaries within which their taxes and jurisdiction operate, ruled through centralised and military power and integrated economies.