This is the word used in history books and in stories set in that time. It was the southern name for the men on the northern side, so it came with hard feeling built in, not as a neutral label. You will meet it most often in the plural, standing for the whole northern army. Wider sources also use it for anyone tied to the Union government of those years, not only the men in uniform. Today this meaning belongs mainly to history and fiction; when people talk about living people, they mean one of the other meanings.

Who counts as a Yankee depends on where the speaker stands. To someone in the South, almost anyone from the North can be one. The word carries a feeling about the person, not only a fact about the map. Said by a friend at a dinner table, it is teasing. Said in a hard voice, it is meant to hurt. People in the North sometimes use it about themselves, and then it sounds proud instead of rude. So the same four letters can be warm or cold, and the voice around them is what tells you which.

This use flattens the whole country into one word. A speaker in London or Sydney is not thinking about North or South; they mean American. The feeling attached is usually not warm, so a writer who picks Yankee over American is often showing some annoyance. Among friends it can be light, the way a nickname is light. Because the feeling is hard to control, American is the safe word when you simply mean where someone is from.

This is the narrowest circle the word draws. People in the northern US often use it this way among themselves, and here it usually works as a plain statement of where someone is from rather than as a put-down. Because it is narrow, it can clash with the wider uses: a person from Chicago is a Yankee to a southerner but not a Yankee in this sense. When you meet the word, the speaker's own home is a good clue to which circle they mean.