Steve Sailer has his famous definition of race:
An extended family that inbreeds to some degree.
I have decided that my contribution to this issue will be my definition of "nation:"
A race that allows for the existence of adopted members.
The U.S. is a good example of this; throughout most of our history, we have turned people from different backgrounds into "Americans." We're not as diverse as the liberals would have us belief, or as they would want, but a person's status as an American is not based on his biological race.
Of course, this is not to say that a nation is a synonym for a country (which is a land mass) or a state (which is the government). You can have nations without a state (e.g. the Kurds), and a state need not be a nation (Iraq). But the term "nation" as opposed to "race" does at least imply the possibility of someone "gaining membership" into the group.
I also decided to contribute a definition of "ethnic group:"
A collection of one or more races sharing some bond that allows for (a) the existence of adopted members, (b) for the adoption of new races into the collection.
Take the example of the Jewish people. The distinct subsets of Jews could well be more different from each other than each is from its Gentile neighbors, but they still are all united under the rubric of Jewishness. Moreover, a person of any race can convert, and therefore be considered Jewish regardless of race.
This is something I touched on earlier, but I thought I would make an explicit definition.
That is all.