What do you call it?
So what do you call a piece of paper A4 for a printer if you already used one side of it. Let me explain. You printed something by mistake but you don't want to waste the paper or you simply ran out of blank paper. So you can turn it upside and print something else one more time. What do you call this paper? In the dictionary I found the word 'dirty copy' but I'm not sure that this is the correct term for this.

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There is no exact word for what you describe (as far as I know). If you deliberately print on both sides of a page then you've made a "double sided copy" or a "2 sided copy", but that doesn't describe your very precise example. In your case I would simply call it "reused paper" or "reprinted paper".

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I agree with Jeremy, "scrap paper" is probably the best word for that. There isn't specifically a word that means one side has been used and one is free, but scrap paper implies it's not a fresh clear piece of paper and can be re-used.

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I wouldn't know what you meant if you said "dirty copy."  I would say that the paper is "used" or I might refer to it as "scrap paper." I think "scrap paper" is probably the most common term.