In the context of air travel and immigration, nationality means the same as citizenship. The US is funny about some US nationals not being citizens, but the rest of the world regards such people as US citizens.
Ok, that makes sense. There does seem to be a difference in the U.S. though. But I looked it up, and you're right that for US passport purposes, both citizen and national would be "USA". Thanks for clarifying, I clearly didn't provide a good legal definition.