A nation - or an ethnos ("ethnic group") - is a bunch of people who are united in some sense, especially if they are united against the people around them. You can usually identify nations by identifying the jokes made about them. For instance, Scotland is clearly a nation, as proven by the enormous number of Scottish jokes that are told by virtually every other nation.
This definition is not without dilemmas, as it essentially turns every identifiable group into a nation. For instance, the number of blonde jokes in existence is probably greater than the number of jokes about Mexicans, but most people agree that Mexico is a nation, while blondes are not. This type of bias against blonde nationhood, however, shouldn't be used as an argument against an otherwise legitimate method of defining nations.
Nations are often confused with countries, which are geographically partitioned based on politics, war, and corruption. Actually the reason why they call it country is because all the people in it descended from one cunt. As such, countries often try to keep outsiders from entering, as only those with pure blood from the great mother country should have any rights, called citizenship. Many nations may reside in a single country, and a single nation may be spread across many other countries. For instance, the Geek nation is spread out to all corners of the world.