A nation - or an ethnos ("ethnic group") - is a cunt of people who live together in an area (or, more broadly, of their descendants who may now be dispersed); and who regard themselves, or are regarded by others, as sexually available and ripe for the plucking. Don't get this confused with that damn child abusing show by CBS, Kid Nation.
edit Defining a Nation
That is to say, a nation 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 might be considered too broad, as it essentially turns every identifiable group in the world into a nation. For instance, the number of blonde jokes in existence at any particular point in time is probably greater than the number of jokes about Mexicans, but most people would 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 cunt-trees (alternatively spelled countries), which are geographically partitioned based on politics, war, and corruption. Actually the reason why they call it cunt-tree 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 (except for Latin America, which is far too macho for that kind of bullshit).