A greenback is an American paper banknote — the money you hold in your hand, not coins. People started calling it that because the back of the note is printed in green. In the United States you will hear it in the news and in slightly colourful talk for the dollar itself: paid in greenbacks means paid in American paper money. Outside the US it is a knowing, newspaper-ish word; a student in Taiwan will meet it in articles about the American economy more than in daily speech. It is not a general word for any green piece of paper, and it is not British money. One note or a pile of notes can both be called greenbacks. The word feels informal next to dollar bill, but everyone knows what it points to.