It is something of a shame that the English language, for all its adaptability, does not have a better demonym for citizens of the United States of America. In English we simply say “Americans”, which works fine in day-to-day usage in the US, but in Spanish this is no good. In Spanish, an “americano” is someone from the Western Hemisphere; “América” is the Spanish version of “the Americas” in English. When I studied in Uruguay and said things like “Americans have a different way of approaching that topic [than the local custom]” they would always comment back that my statement was nonsensical.
In Spanish there are two demonyms for people from the United States of America: “norteamericano” and “estadounidense”. The first suffers from the flaw that its denotation is the main sense of the word, and its connotation is not specific enough. The second suffers from the fact that to bring this into English would result in some kind of gross linguistic awkwardness, so much that it simply wouldn’t be done.
There are two other reasons. When the United States of America were first brought together as a distinct unit, there were no other sovereign states to consider in the Western Hemisphere; the definition sufficed to specify exactly what was meant. In the modern day, “Americans” form the dominant country in the world and don’t feel any specific urgency to get more specific because damn it, you know exactly what we mean!
I see this persisting for a long, long time. Perhaps as demographic, cultural, and economic changes add up over hundreds of years the standard English word will more closely resemble the standard Spanish word. I guess nobody alive right now will ever know.
UPDATE: In response to a Facebook comment, I’m adding this part.
“Gringo” frequently works pretty well—though a Mexican term it’s well understood in other parts of Latin America—and I have no particular objection to it, but it is mildly pejorative and I don’t see that this will become standard on that account. It also doesn’t really apply outside of white people in the USA, and there are many non-white people (by the standard conception of “white” and by more inclusive conceptions). For instance President Obama is not a gringo, but he’s still the most powerful man in the country. As the Hispanic population grows the term will becomes less and less applicable.
“Yankee” refers to a specific cultural group in the early period before the USA existed as a country. Descendants of Yankees are still referred to by this name, but it doesn’t apply to most of the country. Certain regions may not care if this term becomes general, but certain other regions will oppose its adoption. Still no dice.